Writer-director Lonergan's 2000 debut You Can Count On Me, a low-budget indie drama about the fractious relationship between adult siblings was a revelation and one of the best films of its genre and year.
His follow-up has been a long time coming: it was shot seven years ago with a dream cast (small roles are taken by Kieran Culkin and Matthew Broderick) and heavyweight producers (two of whom, Anthony Minghella and Sydney Pollack, died in 2008).
But it became mired in legal suits and counter-suits, mainly because Lonergan wanted a three-hour cut and the studio wanted a two-hour one.
The result, which splits the difference, is not the version Martin Scorsese hailed as a masterpiece in 2006, though since Scorsese was instrumental in finessing the eventual (and commercially catastrophic) release, he was presumably happy with the finished product.
It's hard to share that unbridled enthusiasm: Margaret is too long, by at least half an hour, in part because it has one too many subplots and - to compare it to a fugue - it revisits its principal subject too often, to too little effect.
But that's not to say the film is without interest, even a tragic grandeur. Its central character, played by Paquin, is a privileged Upper West Side teenager named Lisa Cohen (the title's Margaret refers to Gerard Manley Hopkins' 1880 poem Spring and Fall, which it is useful to read before or after going).
A high-school student in a Manhattan over which the shadow of 9/11 still stretches, Lisa wrestles with teen angst; classmates she looks down on; a neurotic actress mother (Smith-Cameron, Lonergan's wife) who's dating again (Reno plays an achingly sincere suitor); and a distant father (Lonergan himself), who's a screenwriter in Hollywood.
When she inadvertently contributes to a dreadful bus accident, she lies to the cops to protect the driver (Ruffalo). But stricken by guilt which she interprets as ethical resoluteness, she spends the rest of the movie trying to make him pay.
As a portrait of a youngster trying to make sense of her moral universe, the film achieves an impressive degree of complexity and nuance, and Paquin's ferociously intense performance is always persuasive.
But it's a struggle spending so long in the company of such an unlikeable protagonist, who oscillates between shrill and whiney.
And the last 20 minutes, as she becomes more desperate for a goal receding further with every minute, are probably more painful than they were meant to be.
Cast: Anna Paquin, Mark Ruffalo, Matt Damon, Kenneth Lonergan
Director: Kenneth Lonergan
Rating: R16 (drug use, sex scenes, offensive language and content that may disturb).
Running time: 150 mins.
Verdict: Too long and shrill but Paquin is great.