Lucas, 40, is the star of a new TV drama based on John Grisham's legal thriller "The Firm."
The actor announced his engagement Friday on "The Tonight Show with Jay Leno," but didn't mention his fiancee's name or occupation.
He later told UsMagazine.com he intends to "get married over Christmas."
"We've been engaged for awhile, but honestly, the show has been a bit of an epic undertaking, to say the least," the celebrity news Web site quoted Lucas as saying. "We want to do it when we can enjoy it and take our time. We don't feel any stress or pressure right now and we want to make sure it stays that way."