The star's critically-acclaimed role in the Coen brothers' No Country For Old Men beat competition from his co-star Tommy Lee Jones, There Will Be Blood's Paul Dano, Philip Seymour Hoffman for Charlie Wilson's War, and Tom Wilkinson for Michael Clayton.
Accepting the honour at the London ceremony on Sunday (10Feb08), he said: "I want to thank the mummy and daddy of, not mine, (but) Joel and Ethan Coen, because without them there would be no film..."
Meanwhile, German drama The Lives of Others triumphed in the Best Foreign Language Film category, after being chosen ahead of movies including La Vie En Rose and The Diving Bell And The Butterfly.
However, The Diving Bell And The Butterfly won gold in the Best Adapted Screenplay category, with the award going to writer Ronald Harwood.
This Is England was also an early winner at the BAFTAs - the Shane Meadows-directed drama, about a group of young skinheads in the U.K. in the early 1980s, beat competition from Atonement, The Bourne Ultimatum, Eastern Promises and Joy Division biopic Control to take home the Best British Film gong - the first award of the night.
Transformers star Shia LaBeouf was also honoured at the ceremony, after beating Sienna Miller, Sam Riley, Ellen Page and Tang Wei to the Orange Rising Star Award.
The honour, which praises up-and-coming Hollywood actors, is the only one of the BAFTAs voted for the public and was established in honour of late casting director Mary Selway, who died in 2004.
Source: WENN