The 'best film' honour capped a strong night for the movie overall, which included important wins for Morgan Freeman as best supporting actor, Hilary Swank as best actress and Eastwood as best director.
"This is the third award cermeony we've gotten to in this category," said producer Albert S Ruddy, "and tonight we won. Every time we started going to this thing I said to myself 'I'm going to enjoy whether I win or lose,' but it's better to win.
"This is a great honour that celebrates the talent of Hilary Swank, Morgan Freeman and the genius of Clint Eastwood."
Million Dollar Baby beat out favourite The Aviator in the category, as well as other contenders Finding Neverland, Ray and Sideways.