Season One

Lilah is a lawyer for the evil law firm Wolfram & Hart, and in her first appearance, she tries (and fails) to persuade Angel to work for her employers. She then works to kill Angel; she is co-architect of a plan to hire renegade Slayer Faith Lehane in order to do just that. Over time, Lilah and another lawyer, Lindsey McDonald, become rivals, competing for the favor of their superiors.

Season Two

The vampire Darla is resurrected as a human by Wolfram & Hart and ultimately turned back into a vampire by Drusilla. Darla and Drusilla massacre a group of Wolfram & Hart lawyers in Holland Manners' wine cellar. Only Lilah and Lindsey are spared, and with only the two of them left on Wolfram & Hart's Special Projects Team, their rivalry reaches an all-time high as they start a power struggle for the vice-presidency of the team, as well as for survival. Eventually, Lindsey is offered the promotion, but he elects to leave the firm, disillusioned by the extent of its evil. He tosses his promotion to Lilah, thereby saving her life.

Season Three

In the third season, Lilah ups the ante in her vendetta with Angel, and blackmails him into freeing a mysterious young man, Billy Blim, from a Hell dimension by torturing Cordelia Chase. In the episode "Billy," the effects of these actions come back to haunt Lilah. As it turns out, Billy is a half-demon misogynist, and has the ability to bring out primordial violence in men simply by touching them. He touches Lilah's newest rival, Gavin Park, causing Gavin to beat her severely. Lilah hides herself and her bruises in her apartment for a time, and refuses to assist Angel when he comes to her for help in sending Billy back to Hell. Later on, Cordelia also comes to ask for aid in ending the evil Lilah helped begin, and Lilah again refuses. "I am not Lindsey McDonald," she states flatly. "I don't switch [sides] whenever it gets tough." In response, Cordelia calls her a "vicious bitch." Lilah shrugs. "So you know me."

"Please. I was you, with better shoes," Cordelia snaps. Still, Lilah asks why she should help them. "[Because] no woman should ever have to go through that. And no woman strong enough to wear the mantle of 'vicious bitch' would ever put up with it."

Lilah gives in, telling her where Billy is going and letting Cordelia go confront him on her own. The final conflict among Billy, Cordelia, and Angel takes place on the runway of a private airport, and Lilah arrives at the eleventh hour to fire the fatal shot, killing Billy and regaining her own sense of self-worth. It is around that time that she also receives a new boss, Linwood Murrow, who joins Lilah in trying to manipulate Angel and kidnap his newborn son, Connor.

Lilah herself investigates the return of Daniel Holtz, an old enemy of Angel's. Holtz has broken away from his partnership with the demon Sahjhan, who wishes to eliminate Angel for his own reasons, and he seeks out Wolfram & Hart. He brokers a deal with Lilah, a deal that ultimately results in Connor and Holtz being lost into the Quor-Toth dimension.

At the end of the third season, Lilah begins sleeping with Wesley Wyndam-Pryce after his betrayal of Angel and ejection from Angel Investigations. She tries to convince him to join Wolfram & Hart, but he continues to refuse those advances.

Season Four

Lilah's relationship with Wesley, which begins as a series of one-night stands, continues throughout the fourth season, through the emergence of The Beast and the destruction of the Los Angeles branch of Wolfram & Hart that results in the death of all employees present except for Lilah, who is able to escape with Wesley's help. Shortly before that time, she kills Linwood, and once more rises in power.

Lilah is killed shortly after the re-emergence of Angelus, stabbed to death by Cordelia (who is under the possession of the being later known as Jasmine) and left for Angelus to drink from. Wesley is forced to behead Lilah, as he believes she could rise again as a vampire. When Wesley prepares to decapitate her body, Lilah appears, standing across from him, and the two of them discuss their relationship and Wesley's regret that he could never save Lilah from her own evil. The vision of Lilah implies that she had loved Wesley, though Wesley refuses to believe it, and with the decapitating swing of his axe, the vision is gone.

As shown earlier in the series with Holland Manners, an employment contract with Wolfram & Hart does not terminate with death, and Lilah returns at the end of the fourth season wearing a scarf around her neck to hide the scar from Wesley's axe. She presents Angel with a tempting offer: full control of the Los Angeles branch of Wolfram & Hart with all its resources. During a tour of the facilities intended to seduce Angel Investigations into accepting the offer, Wesley attempts to burn Lilah's contract in order to give her peace. Though the contract cannot be destroyed, Lilah is touched by the attempt, and tells her former lover in their final scene together, "It means something that you tried."

Angel takes Lilah's offer in an attempt to save his son Connor from a life of misery, and Lilah apparently returns to the Hell division of Wolfram & Hart. In the fifth season, Eve takes over as liaison for the Senior Partners.