[10] 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".

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.

[13] 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.

In 2001 he wrote the play Lobby Hero which premiered off-Broadway at Playwrights Horizons starring Glenn Fitzgerald, Heather Burns, and Tate Donovan.

The film spent over five years in post-production, with Lonergan, the producers and various editors unable to agree on its final cut, resulting in multiple legal disputes.

[16] In August 2014, This Is Our Youth was revived on Broadway starring Michael Cera, Kieran Culkin, and Tavi Gevinson at the Cort Theatre.

Alexis Soloski of The Guardian wrote, "Though still hewing to drama school rules – small cast, single set, neat inciting incident – it has a mature, post-grad voice, sensitive and humane and jammed full of totally splendid crackerjack-with-a-toy-surprise dialogue".

[17] Lonergan wrote and directed the drama film Manchester by the Sea (2016) starring Casey Affleck, Lucas Hedges, and Michelle Williams, which was released to critical acclaim.

David Fear of Rolling Stone said the film proves that Lonergan is "practically peerless in portraying loss as a living, breathing thing without resorting to the vocabulary of griefporn.

[22] In March 2018, a second play of his, Lobby Hero was revived on Broadway, this time starring Chris Evans, Michael Cera, Brian Tyree Henry, and Bel Powley at the Helen Hayes Theatre.

Leah Greenblatt of Entertainment Weekly wrote, "Lobby is still a smart, thoughtful piece of work, fairy-dusted by the starry presence of its celebrated cast".

[24] Later that year, In September 2018 a third show of his, The Waverly Gallery, was revived on Broadway starring Elaine May, Lucas Hedges, Joan Allen, David Cromer, and Michael Cera at the John Golden Theatre.

It feels a privilege to watch this legend transform Lonergan’s meditation on dignity, regret and ungraspable memory into something indelible and lasting.

During this time Lonergan took small supporting roles in projects such as Noah Baumbach's absurdist comedy-drama White Noise (2022) and the Netflix thriller miniseries Ripley (2024).