The son of a doctor and a psychoanalyst, Lonergan was born and raised in Manhattan. While in high school, he was asked to collaborate on a play by his drama teacher, and his interest was sparked, eventually leading him to major in dramatic writing at NYU. After graduation, Lonergan earned a living penning material for industrial productions for clients like Fuji Film. A spec script he penned about a gangster who seeks treatment for depression was eventually optioned in the early 1990s. Although it went through numerous rewrites and had contributions from approximately 14 additional scripters, "Analyze This" (1999) gave him his first on screen credit. By the time that film became a box-office hit, grossing in excess of $100 million, Lonergan had begun to make inroads in the theater.
A member of the Naked Angels theater company, Lonergan garnered attention for his 1993 one-act play "Betrayal By Everyone", staged as part of a festival of short plays at the Met Theater. Featuring Mark Ruffalo in the cast, the play focused on a trio of disaffected, well-off twentysomething New Yorkers. Expanding the work into a full-length piece retitled "This Is Our Youth", he earned further success and notoriety as a purveyor of great dialogue, first with a 1996 Off-Off-Broadway staging, but even more so with the 1998 Off-Broadway version. (In both cases, Ruffalo reprised the role of the slightly vacant slacker named Warren,) With this success coupled with "Analyze This", Lonergan emerged as a writer in demand.
Instead of accepting most of the offers thrown his way, Lonergan instead wrote, directed and co-starred in an independent film that went on to earn numerous kudos. "You Can Count on Me" (2000) centered on the relationship between a single mother who remained in her hometown and her younger brother, a screwed-up wanderer. His script was richly detailed and nuanced, charting the ebb and flow of sibling love and rivalry. While some faulted the lack of visual pyrotechnics, Lonergan proved a strong director of actors, eliciting believable performances from his leads, Ruffalo and Laura Linney, as well as strong supporting turns from Rory Culkin (as Linney's son) and long-time chum Matthew Broderick (as Linney's fussy boss). A hit at the Sundance Film Festival (where it shared top honors with "Girlfight"), "You Can Count on Me" went on to pick up several end-of-year citations, including several for Lonergan's screenplay.
His other 2000 big screen effort, "The Adventures of Rocky and Bullwinkle", did not enjoy as much success as hoped. While Lonergan remained faithful to the spirit of the Jay Ward cartoons, the film mix of live-action and animation didn't impress reviewers or audiences. On the other hand, his stage play "The Waverly Gallery", based in part on the experiences of his grandmother, won high praise from reviewers. Lonergan's crisp dialogue was once again singled out for praise as was the career-topping turn of Eileen Heckart in the lead role. Although audiences stayed away from the piece -- in part because of the misperception that it was a depressing depiction of Alzheimer's disease -- the play indicated that Lonergan was one of the American theater's potent new voices.