"Whoever thought I would ever win in the comedy/musical category?," he said. Witherspoon earlier picked up the best actress award in the same category for her portrayal of Cash's wife, June Carter Cash.
Ang Lee picked up "Brokeback Mountain's" third Golden Globe award Monday night for best director of a feature film.
"It's embarrassing for me to say, I spend so much time making movies, I get too critical to enjoy them," Lee said. "This year I have seen and enjoyed so many of my colleagues films it makes this award so much more special.
"I have been waiting five years to thank someone I forgot last time, my film editor, Tim Squyres," he said. "It's been bugging me all this time."
"A Love That Will Never Grow Old" from "Brokeback" garnered composer Gustavo Santoalalla and lyricist Bernie Taupin the Golden Globe for best original song in a motion picture.