The first season of the television series Angel, the spin-off of Buffy the Vampire Slayer, premiered on October 5, 1999, on The WB and concluded its 22-episode season on May 23, 2000. The season aired on Tuesdays at 9:00 pm ET, following Buffy.

Series co-creator David Greenwalt points out "there's no denying that Angel grew out of Buffy". Several years before Angel debuted, Joss Whedon developed the concept behind Buffy the Vampire Slayer to invert the Hollywood formula of "the little blonde girl who goes into a dark alley and gets killed in every horror movie." The character Angel was first seen in the first episode of Buffy and became a regular, appearing in the opening credits during seasons two and three. After being turned into a soulless, immortal vampire, he became legendary for his evil acts, until a band of wronged Gypsies punished him by restoring his soul, overwhelming him with guilt. Angel eventually set out on a path of redemption, hoping that he could make up for his past through good deeds. In Buffy's season three finale, he leaves Sunnydale for L.A. to continue his atonement without Buffy. Whedon believed that "Angel was the one character who was bigger than life in the same way that Buffy was, a kind of superhero." Whedon has compared the series to its parent, "It's a little bit more straightforward action show and a little bit more of a guys' show."

While the central concept behind Buffy was "High school as a horror movie" in small-town America, co-creators David Greenwalt and Whedon were looking to make Angel into a different "gritty, urban show." Whedon explains "we wanted a much darker show, darker in tone. It is set in Los Angeles because there are a lot of demons in L.A. and a wealth of stories to be told. We also wanted to take the show a little older and have the characters deal with demons in a much different way. Buffy is always the underdog trying to save the world, but Angel is looking for redemption. It's those two things that creatively make the shows different."

Whedon and Greenwalt prepared a six-minute promotional video pitch, often called the "Unaired Angel pilot" for the WB Network. Some shots from this short were later used in the opening credits.

Early during the life of the series, some effort was made to slightly soften the original concept. For example, scenes were cut from the pilot episode, "City of," in which Angel tasted the blood of a murder victim. The episode that was originally written to be the second episode, "Corrupt", was abandoned altogether. Writer David Fury explains, "The network was shocked. They said 'We can't shoot this. This is way too dark.' We were able to break a new idea, we had to turn it over in three days." Instead the tone was lightened, and the opening episodes established Angel Investigations as an idealistic shoestring operation.

A first draft script reveals that Angel was originally intended to include the character Whistler, played by Max Perlich, who had already featured in the two-part Buffy season two finale. In an interview, Perlich said, "I never got called again. If they had called, I would have probably accepted because it was a great experience and I think Joss is very original and talented." Instead, the producers created a Whistler-like character, Doyle. Cordelia Chase, also from the original Sunnydale crew, joined Angel and Doyle.

At the end of the third season of Buffy, the 242-year-old, re-ensouled vampire Angel left Sunnydale to give teenage Slayer Buffy Summers the chance to live a normal life. Angel is now living in the big city of Los Angeles. With Buffy gone Angel is now completely cut off from society, every day getting closer and closer to giving in to his vampiric hunger. It's not until he befriends the half demon and fellow Irishman Doyle, who is sent visions of people in trouble by The Powers That Be, and fellow Sunnydale resident Cordelia Chase, who grounds Angel's life in the humanity around him, that he truly begins to take charge of his life and seek atonement for his past sins. He sets up his own detective agency, Angel Investigations and begins to "help the helpless".

Early in the season, Angel befriends a detective named Kate Lockley who at first helps and befriends Angel but turns on him after she finds out what he truly is. As the season comes to a close, Angel gains a semi-ally in the form of a street smart vampire hunter named Charles Gunn who will officially join the team early in the next season. Throughout the year Angel and co. are frequently set upon by the demonic law firm Wolfram & Hart, and the antics of two up and coming lawyers Lindsey McDonald and Lilah Morgan, who have secret plans concerning the vampire with a soul and the role he plays in the forthcoming apocalypse.