Doyle was born in 1970 in Dublin, Ireland, to a human mother and a Brachen demon father. His demonic genes didn't physically manifest themselves until 1991, when he was 21 years-old. At that time, Doyle was a third grade teacher and a soup kitchen volunteer married to Harriet, the love of his life. He didn't take the news of this revelation very well, and his marriage disintegrated. Doyle hid behind the flimsy veneer of a ne'er-do-well hustler, seemingly more interested in where his next drink was coming from than helping others.

Later, he was approached by a fellow Brachen demon, Lucas, who told Doyle that the Scourge, a militant group of pureblood demons, was after all half-breeds and begged for Doyle's help. Doyle turned Lucas away, believing that this wasn't his problem. Soon after he received a vision, which he described as "splittin' migraines that come with pictures," in the throes of which he thought he was having a stroke. The vision showed him a group of massacred Brachen demons. Doyle searched the city to find out if what he had seen was real--it was. These visions, which come from The Powers That Be, are what led Doyle to Angel. As Doyle says, "We all got something to atone for," and therefore the two join forces to fight evil in Los Angeles. Once Cordelia Chase joins the team, Angel Investigations is officially formed.

Doyle soon falls for Cordelia, but is afraid she will reject him upon finding out about his demonic heritage. He also form a close brotherly bond with Angel, with them sharing the same ethnicity and their half-demonic heritages. However, the past comes back to haunt him when the Scourge returns. This time they are after the Listers, another tribe of human/demon hybrids. During the battle, Doyle proves himself a reluctant hero, possessing a kind heart and great strength of character. He dies in Angel's place, sacrificing his life to save his friends, the Listers, and the city of Los Angeles. Not only that, Doyle fulfills the Listers' prophecy of the "Promised One," the bringer purported to save them from the Scourge in the last days of the 20th century. Before Doyle dies, Cordelia learns of his demonic traits and tells him that it doesn't matter to her. The two share a passionate kiss; a blue ribbon of electricity passing between them as he gives her his visions, which change the course of her life considerably.

In the fifth season of Angel, Lindsey McDonald uses Doyle's name in an attempt to control Spike, claiming to fulfill the role for Spike that the original Doyle filled in Angel's life. When they learn what Lindsey has been calling himself, Cordelia and Angel are understandably angered by the misuse of their friend's identity.