Belfour played for 3 seasons in the Manitoba Junior Hockey League with the Winkler Flyers, helping the team to a first-place finish in 1986. As the starting goalie for the top team, Belfour finally received some notoriety and he joined the North Dakota Fighting Sioux for the 1986–87 season. The 21-year-old Belfour was a freshman, older than many of the upper-classmen on his team. He won 29 games that year, helping UND set a new NCAA record with 40 wins on the year and win the National Title. After the season Belfour, as an undrafted player, was able to sign with any team and signed with the Chicago Blackhawks.

For his first professional season, Belfour played for the Saginaw Hawks of the IHL. He won 32 games for the team and helped them reach the IHL semifinals. The following season saw Belfour split his time between the IHL and NHL, playing 23 games with the parent club. He was returned to Saginaw where he helped the team to a 2nd-place finish in the conference but faltered in the postseason and the Hawks were bounced in the first round.

Rather than return to Saginaw, Belfour played the 1989–90 season with the Canadian national men's hockey team. He was recalled by the Blackhawks for their postseason run and produced a 4–2 record with a 2.49 GAA, far better numbers than the other two Blackhawk netminders.

The next season, 1990–91, Belfour became the starting goalie, and had a strong rookie season. He notched 43 victories in 74 games (both NHL rookie and Blackhawk team records), finished the season with a 2.47 GAA and 4 shutouts. He also led the league in Save% (.910). It was the last time a goalie led the league in Wins, Save%, and GAA until Carey Price achieved the feat in the 2014–2015 season.^[4] For his success, he received the Calder Memorial Trophy for outstanding play by a rookie, and is the first person to receive the award under the Makarov Rule because he was a year under the new cutoff age of eligibility (26), the Vezina Trophy for best goaltender and the William M. Jennings Trophy for fewest team goals-against. He was also a finalist for the Hart Memorial Trophy as the league's most valuable player, the first rookie goaltender to do so. He would win the Vezina Trophy and Jennings Trophy again in 1993, and the Jennings Trophy once more with Chicago in 1995.

Belfour helped lead the Blackhawks to the 1992 Stanley Cup Finals in the 1991–92 season, where they met the defending champion Pittsburgh Penguins. In Game 1, Chicago had a 4-1 lead with 8:24 remaining in the second period. However, the Penguins would score twice in the span of a minute to narrow it to 4-3 for the final period. Jaromír Jágr managed to score the tying goal with five minutes remaining, before a late one-man advantage for the Penguins proved key with Mario Lemieux putting a shot rebound past Belfour to give Pittsburgh the go-ahead lead with thirteen seconds left. In total, Belfour had allowed five goals on 39 shots. After a 3-1 loss in Game 2, the series moved to Chicago, where one successful shot meant everything. Kevin Stevens scored with 4:34 left in the first period off Belfour, and Penguins goaltender Tom Barrasso bested Belfour with no goals allowed on 27 shots as Pittsburgh now was one win away from the Cup. Game 4 was a rollercoaster for all involved, especially Belfour. Four goals were scored in the first seven minutes of the game, with each team getting two. After Stevens scored on a wrist shot over Belfour (the second goal on four shots), he was taken out by head coach Mike Keenan for Dominik Hašek. Hašek went 21-for-25 in saves as Pittsburgh rallied late again to win the game and the Finals. On August 7, Hašek was traded to the Buffalo Sabres, where he received more playing time.^[5]

By the 1995–96 season, tension was forming between Belfour and backup goalie Jeff Hackett. By January 1997, reports had surfaced that the two had an incident near a locker room that saw Belfour call Hackett "nothing more than a backup", although the two (along with their coach) asserted that they were more competitive with each other rather than having friction.^[6] A free agent after the season, Belfour was traded to the San Jose Sharks on January 25 for three players (right wing Ulf Dahlén, defenseman Michal Sýkora, goalie Chris Terreri) and a conditional second-round draft pick in the 1998 NHL entry draft after turning down an extension offer ($3.3 million) that would've raised his current rate ($2.75-million). Right after the trade, Hackett received a three-year extension deal.^[7]

Belfour finished his tenure with the Blackhawks ranking among the team leaders in many goaltending categories. Belfour finished third among all Blackhawk goalies in games played (415) and wins (201) in both categories ranking behind Hall of Famers Tony Esposito and Glenn Hall. Belfour also ranks fourth in shutouts (30), and second in assists (17). Belfour easily ranks as the Blackhawks' goalie leader in penalty minutes, with 242. Esposito, who played in more than twice as many games and minutes as Belfour, had only 31.

Following a dismal half-season with the Sharks, Belfour signed as a free agent with the Dallas Stars on July 2, 1997. During the season, Belfour played 61 games and had a 1.88 GAA as his team won the Presidents' Trophy and made it to the Western Conference Finals only to lose to the Detroit Red Wings.

The next season, the Stars repeated their regular season championship and Belfour won his fourth William M. Jennings Trophy. In the playoffs, Belfour won duels against past Vezina- and Stanley Cup-winning goaltenders Grant Fuhr and Patrick Roy, respectively. The Stars won the Stanley Cup, beating the Buffalo Sabres in six games, capped by an incredible goalie duel against former backup Dominik Hašek that ended in a 2–1 win in the third overtime. Belfour made 53 saves to Hašek's 50, and for the entire Finals, had a 1.26 GAA to Hašek's 1.68.

Belfour backstopped his team to another consecutive finals appearance, winning his second seven-game Western Conference final duel against the Colorado Avalanche's Patrick Roy. The Stars lost the Cup in double-overtime to the New Jersey Devils. Belfour had 4 shutouts in that playoffs, including a triple-overtime blanking of the Devils in game five of the finals series.

During the 2001–02 season, the Stars began to play poorly and there was a falling out between then-Stars coach Ken Hitchcock and GM Bob Gainey.^[8][9] After a poor season, the Stars decided not to re-sign Belfour and named Marty Turco the starting goalie for the next season.

On July 2, 2002, Belfour signed as a free agent with the Toronto Maple Leafs after then Leafs goaltender, Curtis Joseph, chose to sign with the Detroit Red Wings.^[10] Belfour rebounded after a dismal season with the Stars, winning a franchise-record 37 games and helping his new team finish second in the Northeast Division. His 2.26 GAA ranked 11th in the league. During the season, he was invited to play in the mid-season All-Star Game in Florida, but a back injury forced him to miss the event. On April 1, he earned his 400th career win in a match against the Devils.^[11] In the playoffs, Belfour posted a 2.71 GAA and a .915 Save% in seven games in an opening-round loss to the Flyers. On April 16 in Game Four at the Air Canada Centre, Ed made 72 saves before losing 3–2 on an overtime goal by Mark Recchi. Belfour finished as runner-up for the Vezina Trophy, won that year by the Devils' Martin Brodeur.

In 2003–04, he posted a 34-19-6 record in 59 games as the Maple Leafs finished fourth overall in the conference standings. He recorded a 2.13 GAA and a .918 save percentage along with ten shutouts. On April 3 in the final game of the season, Belfour posted a 6–0 shutout over the Senators to secure home ice advantage in the opening round of the playoffs. That shutout gave him 10 on the season, setting a new personal best. In the playoffs, Belfour posted three shutouts in the opening round against the Senators, setting a record for shutout streaks in a series. However, in the second round, former teammate Jeremy Roenick eliminated the Leafs by putting a game 6 overtime goal past Belfour.

Belfour did not play during the NHL lockout in 2004–05, instead taking a minority stake in the projected Dallas Americans team in the proposed revival of the World Hockey Association while recovering and rehabilitating himself from primarily back-related injuries. The team had folded by October, 2004.^[12]

On November 28, 2005, Belfour won his 447th career NHL game, moving him into a tie with Terry Sawchuk for 2nd place in career wins.^[13] Ed made 34 saves in the 2–1 win over the Florida Panthers.^[13]

On December 19, 2005, Belfour moved past Sawchuk with a 9–6 win over the New York Islanders at the Air Canada Centre.^[14] He was honoured in a special pre-game ceremony on December 23, 2005, before a game against the Boston Bruins at the Air Canada Centre; the Leafs went on to win the game. At the end of the 2005/06 season, Belfour had a record of 457-303-111 in the regular season, and 88–68 in the playoffs.

On July 1, 2006, Maple Leafs General Manager John Ferguson, Jr. released Belfour to free agency after posting a 22-22-4 record and a 3.29 GAA.^[15]

On July 25, Belfour signed with the Florida Panthers. In October 2006, Alex Auld was injured while the two goalies were horsing around.^[16] On February 13, 2007, Belfour tied Hall of Famer Tony Esposito for eighth place on the career shutout list with his 76th in the Panthers' 1-0 blanking of the Montreal Canadiens. Later in the season, another injury to Alex Auld gave Belfour the chance to become starter. He started 27 consecutive games, a record for the Panthers. Belfour regained his form during the 2006–07 season by posting a 2.79 GAA, .902 save percentage, and 1 shutout in 57 games.^[17]

On August 27, 2007, it was announced that Belfour would play with Leksands IF in the Swedish second division. (HockeyAllsvenskan).^[18] Belfour's signing created much fanfare in the following months. He played his first professional game outside of North America in 18 years on October 31, 2007 with a 4–1 win over Sundsvall. Belfour followed up this game with a shutout streak lasting for 251 minutes, a club record in Leksand. He also broke the record for most shutouts during a whole season with 7.

During the division round, Belfour had a GAA of 1.79, which was the best of all goalies in Allsvenskan. During the playoffs, he had a GAA of 2.59 and a save percentage of .911.

Throughout his career, Belfour has worn masks featuring an eagle on either side of his helmet. When asked why an eagle, he stated "I've always liked the eagle as a bird. It is a strong figure representing individuality, leadership, confidence, and outstanding vision. Its hunting and aggression are characteristics I admire, so when I was thinking of what I wanted on my mask, the eagle was a natural choice". Belfour's eagle has changed dramatically, from a rough Native looking style in Chicago, to a fierce competitive image in Dallas, while the background always features his current team's colours. On the chin, there is an image of the logo for the Make-a-Wish Foundation, a charity very close to his heart, and the back plate highlights his passion for speed and restored cars. The car on the back is a 1941 Willys, along with the words Carman Racing, which is the name of Belfour's car customization and restoration shop in Freeland, Michigan. Upon seeing Belfour's eagle mask for the first time, Mike Keenan, his head coach when he started in the NHL, nicknamed him "The Eagle".^[19]

Belfour was selected to represent Canada at the 1991 Canada Cup Championship as the backup goaltender and was included in the squad for the 2002 Winter Olympic Team. In February 2002, Belfour won an Olympic gold medal with the Canadian men's hockey team. Although he didn't play in any of the Olympic Games in Salt Lake City, he did add depth in goal to the strong Canadian team backing up Curtis Joseph and Martin Brodeur.

Early in the 2000–01 season, on October 20, Belfour pleaded guilty to a misdemeanor charge after a woman called police when the drunken Belfour began intimidating the woman and screaming at her in a Dallas hotel room. While under arrest and being transported the local division, he allegedly offered Dallas police officers $1 billion for his release without charges. He apologized to the Dallas Stars organization and police officers involved and was fined $3,000 for resisting arrest.^[20]

Late in the 2006–07 season, Belfour, along with Panthers teammate Ville Peltonen, was arrested on April 9 outside a South Florida nightclub and was charged with disorderly intoxication and resisting an officer without violence. He was released the same day from Miami-Dade County jail on $1,500 bond.^[21]

On January 28, 2020, Belfour was arrested and booked into the Warren County Regional Jail after an early morning incident at the Kentucky Grand Hotel and Spa in downtown Bowling Green, Kentucky.^[22] Belfour was charged with third-degree criminal mischief and alcohol intoxication in a public place.^[22] Belfour damaged hotel property, and was "manifestly under the influence of alcohol to a point he was a danger to himself and others," according to the police report.^[22]

In his post-playing career he was inducted as a member of Manitoba Sports Hall of Fame^[23] The Manitoba Junior Hockey League also awards a trophy named after Belfour to its top goaltender each season.^[24]

• List of NHL goaltenders with 300 wins