Kenneth Lonergan (born October 16, 1962) is an American playwright, screenwriter and film director, and actor. He is known for his works which explore complex emotional and interpersonal dynamics. He has received several awards including an Academy Award and a BAFTA Award as well as nominations for three Golden Globe Awards, a Pulitzer Prize, a Laurence Olivier Award, and three Tony Awards.

In 1982 Lonergan wrote his first one-act play, The Rennings Children. He then earned acclaim for a string of off-Broadway plays, including This is Our Youth (1996), The Waverly Gallery (2000), and Lobby Hero (2001), which were later revived on Broadway, earning him nominations for three Tony Awards for Best Revival of a Play in 2015, 2019, and 2018, respectively. For his work on The Waverly Gallery, he was selected as a finalist for the Pulitzer Prize for Drama.

He directed and wrote the drama film Manchester by the Sea (2016) which won him the Academy Award for Best Original Screenplay as well as a nomination for the Academy Award for Best Director. He was previously Oscar-nominated as a writer for his first film You Can Count on Me (2000) and as a co-writer for Martin Scorsese's historical epic film Gangs of New York (2002). He also wrote and directed the psychological drama film Margaret (2011), and adapted the E.M. Forster novel Howards End into the 2017 miniseries of the same name.

Lonergan was born in The Bronx, New York City to a psychiatrist mother and physician father. His mother is Jewish, and his father was of Irish descent. Lonergan began writing in high school at the Walden School, a now defunct, highly progressive private school in Manhattan with a strong drama program.

His first play, The Rennings Children, was chosen for the Stephen Sondheim-founded Young Playwrights, Inc. Young Playwright's Festival in 1982 while he was still an undergraduate. Lonergan matriculated to Wesleyan University, where he trained as a playwright and director. He graduated from the NYU Playwriting Program. He is an alumnus of HB Studio in New York City. Lonergan is a founding member of Naked Angels. After graduating from NYU, Lonergan worked as a speechwriter for the Environmental Protection Agency. He also wrote industrial shows (long-play commercials) for clients such as Weight Watchers.

Lonergan's first theatrical success came with the play This Is Our Youth (1996) which opened off-Broadway Intar Theatre by The New Group. The play revolved around three young adults navigating friendship, identity, and disillusionment in 1980s New York City. The cast included Mark Ruffalo, Josh Hamilton, and Missy Yager and was directed by Mark Brokaw. Greg Evans of Variety wrote, "What we have is two hours of conversation among the three characters, and as finely written as the dialogue is" adding, "Lonergan’s ear is unfailing".

It was followed by what is considered Lonergan's breakthrough play, The Waverly Gallery (2000), based on his grandmother's Greenwich Village gallery. The play revolves around a family reacting to an elderly woman in early stages of dementia. The original cast included Eileen Heckart, Josh Hamilton, Maureen Anderman, and Mark Blum. It opened Off-Broadway at the Promenade Theater in March 2000 directed by Scott Ellis. Charles Isherwood of Variety declared, "Anyone who has lived through this sad process with a relative will be awed by the verisimilitude of both Heckart's acting and Lonergan's writing." For his work he was named as a finalist for the Pulitzer Prize for Drama.

Lonergan's film career began with his screenplay for the crime comedy Analyze This (1999) starring Robert De Niro and Billy Crystal as a gangster and his therapist, respectively. Lonergan directed his own screenplay for You Can Count on Me (2000) which was executive produced by Martin Scorsese. The film starred Laura Linney and Mark Ruffalo, and film earned Lonergan a nomination for the Academy Award for Best Original Screenplay. He was subsequently offered a job writing the live-action comedy The Adventures of Rocky and Bullwinkle (2000).