Paul Haggis wrote the screenplay on spec after being fired from the television series Family Law, and it took four years to sell it, with the project stuck in development hell for years.

Several studios rejected the project even after Clint Eastwood signed on as actor and director, and even Warner Bros., Eastwood's longtime home studio, would not initially agree to a $30 million budget.

Eastwood persuaded Lakeshore Entertainment's Tom Rosenberg to put up half the film's budget and handle foreign distribution, with Warner Bros. contributing the rest.

The titular phrase 'million dollar baby' was originally used as an insult during pre-fight publicity by Sonny Liston against Muhammad Ali, who was considered an underdog at the time.

Eastwood shot the film in less than 40 days between June and July 2004, filming in Los Angeles and on sets at Warner Bros. Studios.

Eastwood cast his own daughter, Morgan Colette, in a cameo as a girl who waves to Hilary Swank's character at a gas station.

Eastwood had initial concerns about Hilary Swank's small physique for the boxer role, saying he needed to get her 'trained up' and looking like a fighter.

To prepare for her role, Hilary Swank trained nearly five hours a day and gained 19 pounds (8.6 kg) of muscle under professional trainer Grant L. Roberts, developing a potentially life-threatening staph infection from foot blisters that she hid from Eastwood because she felt revealing it would be out of character for Maggie.

The Gaelic nickname 'Mo Cuishle' given to Maggie's character derives from the phrase 'a chuisle mo chroí,' meaning 'O pulse of my heart,' and its use in the film sparked increased public interest in the Gaelic language.

The film's ending sparked significant controversy, with disability rights activists protesting in cities including Chicago and Berkeley, arguing it reinforced harmful stereotypes about quality of life with disabilities.

Million Dollar Baby was the first Best Picture Oscar winner released on a high-definition optical disc format in the United States, arriving on HD DVD in April 2006 before Crash became the first Best Picture winner released on Blu-ray.