LOS ANGELES • Aretha Franklin, who died in August, sued repeatedly over the years to block its release.
But Amazing Grace - capturing what is considered to be her most transcendent gospel performance - will have its world premiere on Monday in New York at Doc NYC, a festival dedicated to non-fiction cinema.
The 87-minute documentary comes 46 years after it was shot.
"Her fans need to see this film, which is so pure and joyous," Ms Sabrina Owens, Franklin's niece and executor of the Franklin estate, said. "And the world needs to see it. Our country, it's in such a state right now."
Alan Elliott, one of the film's producers, said it would likely arrive in wide release in January.
Amazing Grace was shot by film-maker Sydney Pollack over two nights in 1972 at the New Temple Missionary Baptist Church in the Watts neighbourhood of Los Angeles, as Franklin recorded an album that became one of the best-selling gospel records of all time.
Anchored by an 11-minute version of Amazing Grace, the record includes definitive interpretations of songs like Mary Don't You Weep, a slavery-era spiritual.
Mick Jagger sat in a pew towards the back.
But the film recording was mishandled. Pollack, who died in 2008, failed to use clapper boards, a crucial tool in matching sound with filmed images in a pre-digital era.
And he had 20 hours of raw footage to sync. Frustrated film editors at Warner, which financed the shoot, ultimately gave up, having missed the 1972 release of the Amazing Grace album.
Pollack turned to a new directing project, The Way We Were, starring Barbra Streisand and Robert Redford. And the Amazing Grace negatives began to gather dust in the Warner vaults.
Elliott, who had been obsessed with the lost footage since working as a music executive in the mid-1980s, ultimately persuaded Warner to sell him the reels in 2007. He mortgaged his house.
By 2010, digital technology had evolved to a point that syncing film and sound was finally possible.
As a planned release date approached in 2011, however, Franklin sued Elliott for using her likeness without her permission. That started years of legal wrangling, with the singer blocking Elliott and the Telluride Film Festival from showing Amazing Grace in 2015 and 2016, even after deals for her compensation seemed to have been worked out.
Pundits speculated that the release of the movie, which ends with a young Franklin performing Never Grow Old, was simply too difficult for the ailing singer to confront.
Legal clearance finally came after Ms Owens invited Elliott to her aunt's funeral in Detroit.
Movies built around music have scored with audiences. Over the weekend, the Freddie Mercury biopic Bohemian Rhapsody topped the North American box office.
And the dramatic late arrival of Amazing Grace is sure to shake up the race for best documentary at the Oscars. Its producers may also try for a best picture nomination.
"Aretha would want us going for a best picture," Elliott said. "And she'd want to win too."