The agency was founded by Angel sometime between the first and second episodes of Season One. Angel makes friends with Doyle, a half-demon who's been given visions by higher beings of people in need of Angel's help. After they successfully rescue Sunnydale pal Cordelia Chase, she joins them and encourages them to front their operations with a detective agency. Cordelia also coins their slogan, "We help the helpless."

Half-way through season one, Doyle passes his visions on to Cordelia just before his heroic death. Another old Sunnydale friend, self-styled "rogue demon hunter" and former Watcher Wesley Wyndam-Pryce, joins Angel Investigations soon thereafter. At the end of season one, street tough vampire hunter Charles Gunn also joins AI, adding much-needed muscle to their group. Since the beginning, AI has been operating out of Angel's residence, a basement apartment beneath ground floor office space comprised of an inner executive office and an outer reception area. At the end of the first season, however, an agent of the evil law firm Wolfram and Hart bombs the site to rubble.

Cordelia's residence is pressed into service as AI headquarters unitl shortly into the second season, when AI moves to a new location, the Hyperion Hotel. Now empty and all but abandoned, the Hyperion is where Angel stayed, until 1952, during a prior stint in Los Angeles. Half-way through season two, Angel fires his three colleagues after escalating disagreements about his growing moral corruption. Instead, while Angel deals with his private issues, Gunn, Cordelia, and Wesley continue running AI without him in a rented office. Eventually, Angel rejoins them and their base of operations returns to the Hyperion Hotel. During a story-arc in the demon-dimension Pylea, home to friend Lorne, they rescue Winifred Burkle, a brilliant former physics student, who joins the agency at the start of season 3.

At the end of season 4, the agency is dissolved when Angel assumes control over the L.A. branch of their former enemy Wolfram & Hart.

Aforementioned karaoke-demon Lorne, and Angel's son Connor, are also affiliated with the agency, although it is unclear whether they were actually employees.