Backes played three seasons of prep hockey with Spring Lake Park High School in Spring Lake Park, Minnesota, graduating in 2002. As a senior, he was named All-Conference, All-Metro, All-State and was a finalist for the 2002 Minnesota Mr. Hockey award, losing to Gino Guyer. He was joined on the Star Tribune's All-Metro First Team with future college teammate Travis Morin of the Dallas Stars. His high school team included future Minnesota Wild forward Jarod Palmer. His number 5 has been retired by the school and hangs at Fogerty Arena in Blaine, Minnesota.^[1]

After spending parts of two years in junior hockey with the Lincoln Stars of the USHL, Backes was selected in the second round, 62nd overall, by the St. Louis Blues in the 2003 NHL entry draft. Prior to joining the Blues, he played three seasons of college hockey at Minnesota State University, Mankato, where he was named to the 2005–06 Men's RBK Division I West All-America Second Team and the 2005–06 ESPN The Magazine Academic All-America First Team.^[2] Immediately after his third and final college season, he was signed by the Blues and assigned to the Peoria Rivermen of the AHL for the final 12 games of the 2005–06 season.

Following his first NHL training camp in September 2006, Backes was reassigned to the Rivermen, remaining in the AHL for the start of the 2006–07 season. Several months into the campaign, he was called up by the Blues and played his first NHL game against the Pittsburgh Penguins on December 19, 2006. Forty-four seconds into the game, Backes registered his first NHL point, a second-assist on a Doug Weight goal, giving the Blues a 1–0 lead. St. Louis went on to win the game 4–1, while Backes recorded 10 minutes of ice time.^[3] The following game, two days later, Backes scored his first NHL goal 10 minutes and 47 seconds into the first period versus the Los Angeles Kings. The goal came on a backhand shot past Kings goaltender Dan Cloutier, helping the Blues to a 5–2 win.^[4] A month into his initial stint with the Blues, he was reassigned to the Rivermen on January 21, 2007, for three days, after which he was recalled. Spending the remainder of the season with St. Louis, he completed his rookie season with 23 points (10 goals and 13 assists) in 49 games, while also recording 13 points (10 goals and three assists) in 31 games with Peoria. Among NHL rookies who had played at least half the season, Backes ranked 13th in points-per-game at 0.47.^[5]

In 2007–08, he completed his first full season in the NHL, improving to 31 points (13 goals and 18 assists). Becoming a restricted free agent in the off-season, Backes signed a three-year, US$7.5 million offer sheet with the Vancouver Canucks on July 1, 2008. The Blues quickly matched the offer, keeping Backes in St. Louis through the 2010–11 season.^[6] With a new contract, Backes tallied 54 points, including a career-high 31 goals, in 2008–09, finishing second behind former Bruins winger Brad Boyes among Blues scorers.^[7] He added 165 penalty minutes while playing in all 82 games for the first time in his career. During the season, he scored a career-high four goals on April 2, 2009, in a 5–4 win over the Detroit Red Wings.^[8] Backes' efforts helped the Blues reach the playoffs for the first time since 2004. Matching up against the Vancouver Canucks in the first round, the Blues were swept in four games. Backes recorded a goal and two assists in that span.

In 2009–10, Backes' offensive production decreased to 17 goals and 31 assists for 48 points in 79 games. Following the retirement of Keith Tkachuk in the off-season, Backes took on the role of the alternate captain for 2010–11, the last season of his contract. A month into the campaign, on November 12, 2010, Backes signed a five-year, $22.5 million contract extension, keeping him with St. Louis through the 2015–16 season.^[9] Leading the Blues in scoring midway through the season, he was named to his first NHL All-Star Game in 2011.^[10] As the NHL adopted a draft to determine the teams for the first year, Backes was selected by Team Staal 33rd overall among 36 players. He recorded three assists in a losing effort, as Team Lidstrom won 11–10. Backes completed the season matching his personal best total of 31 goals, while adding 31 assists for a career-high and team-leading 62 points over 82 games. With a +32 rating, Backes came within one point of leading the league in plus-minus, behind Boston Bruins defenseman Zdeno Chára.

Late in the 2010–11 season, Blues captain Eric Brewer had been traded away to the Tampa Bay Lightning, leaving the leadership position vacant for the remainder of the campaign. During the off-season, on September 9, 2011, Backes was chosen to succeed Brewer, becoming the 20th captain in team history.^[11] He scored 24 goals to go with 30 assists in 82 games played during the 2011–12 NHL season.

With 21 goals and 24 assists during the 2015-16 season, Backes would help lead the Blues to their first Western Conference Finals appearance since 2001. During the 2016 Stanley Cup Playoffs, he scored seven goals with seven assists.

After ten seasons with the Blues organization and leading the club as captain for the last five seasons, Backes left as a free agent after the 2015–16 season. On July 1, 2016, Backes agreed to a five-year, $30 million deal with the Boston Bruins.^[12]

On November 22, 2016, the Bruins as a team reached the milestone of 20,000 goals all-time in NHL play, dating back to their 1924–25 debut season when left winger Smokey Harris scored the Bruins' first-ever goal on December 1, 1924;^[13] Backes' power play goal for the Bruins got them to the 20,000th-team goal total plateau in a 4–2 home rink loss to his previous team, the St. Louis Blues. The 20,000th goal plateau has only been attained by one other NHL team—the Bruins' top rival—the Montreal Canadiens.^[14]

At the beginning of the 2017–18 season, Backes had been diagnosed with a diverticulitis infection, and missed the first five games of the regular season, but was able to play again by October 19, the night of a home game against the Vancouver Canucks. However, a re-evaluation shortly thereafter, with Backes' most recent game on October 30, on the road against the Columbus Blue Jackets, revealed that an operation to remove part of his colon would be necessary; it was carried out on November 2, with a projected eight-week recovery time.^[15] Backes was suspended for three games on March 7, 2018, due to interference against Detroit Red Wings player Frans Nielsen, during a game on March 6, 2018.^[16] Following an accidental hockey-skate cut above one knee suffered in a March 17, 2018 Bruins road game by the Tampa Bay Lightning's Yanni Gourde,^[17] Backes required 17 stitches to close the wound, and needed to stay off the ice for it to heal - Backes returned against the Lightning in a home game for the Bruins on March 29.^[18] Ahead of his return on March 29, Backes was selected as the Bruins' nominee for the Bill Masterton Trophy, which is awarded to the player who "best exemplifies the qualities of perseverance, sportsmanship and dedication to hockey."^[19]

On December 27, 2018, Backes delivered an illegal hit to the head of New Jersey Devils' forward Blake Coleman. He was assessed a minor penalty on the play. The following day, Backes was suspended for three games. As a repeat offender, Backes forfeited $219,512.19.^[20] In the 2019 Stanley Cup Finals, Backes would faceoff against the team that he once captained, the St. Louis Blues. The Bruins would ultimately lose to the Blues in seven games, one win short from winning the Stanley Cup. Backes appeared in 15 games during Boston's run to the Finals, tallying two goals and three assists.

On February 21, 2020, the Bruins traded Backes, prospect Axel Andersson and a 2020 first-round pick to the Anaheim Ducks in exchange for Ondřej Kaše.^[21]

On May 5, 2021, Backes played his final NHL game against his former team, the St. Louis Blues, in which he received a standing ovation.^[22]

On September 9, 2021, Backes signed a one-day contract with the St. Louis Blues in order to officially retire as a member of the team.^[22][23]

Following his rookie NHL season, Backes was named to the United States' national team for the 2007 IIHF World Championship.^[24] He played in seven games, contributing a goal, two assists and six penalty minutes, as the U.S. finished in fifth place. He again suited up for Team USA at the 2008 IIHF World Championship,^[24] contributing an assist and 35 penalty minutes over six games as the U.S. finished sixth. Competing for a third consecutive year at the World Championships in 2009,^[24] Backes registering a goal and four assists in nine games. The U.S. placed fourth, losing to Sweden 4–2 in the bronze medal game.^[25]

On January 1, 2010, Backes was named to the United States' roster for the 2010 Winter Olympics in Vancouver.^[26] He recorded a goal and two assists over six games, helping the United States to the gold medal game. They were defeated 3–2 by Canada in overtime, earning silver.^[27]

On January 1, 2014, Backes was named to the United States's roster for the 2014 Winter Olympics along with St. Louis teammates Kevin Shattenkirk and T. J. Oshie.^[28] Although the U.S. team did not win a medal, the forward played in six games, scored three goals, had one assist, and four points — while accumulating six penalty minutes — in Sochi.

Backes and his wife Kelly have two children. Backes is a Christian.^[29][30]

Backes got his private pilot's license in 2012 and has flown many planes since. He graduated with an applied organizational studies degree with a nonprofit leadership emphasis from Minnesota State University, Mankato in 2014, and had formerly studied to become an electrical engineer.^[31]

Backes is a passionate advocate for animal welfare, and has had pets since he was a child and when he was struggling to get a break in the NHL. Together with his wife, Kelly, he is highly active in animal rescue efforts and pet adoption drives.^[32] In November 2013, they founded Athletes for Animals, an organization to raise funds and awareness for rescuing and protecting the welfare of homeless pets.^[33]

On April 4, 2017, six cattle that escaped slaughter in north St. Louis were put under the care of The Gentle Barn, an animal sanctuary with facilities in California and Tennessee with David and Kelly helping provide support to the community based GoFundMe campaigns created for their freedom.^[34]