Lehman's performance was his fourth round in the 60s at the event. He ended at 14-under 274, earning his third career Champions Tour major title.
The 53-year-old became to first senior golfer to successfully defend a major championship title since Allen Doyle did so in 2005-06 at the U.S. Senior Open.
Sharing second place at minus-12 were Chien Soon Lu and Bernhard Langer (both carded 6-under 66s).
Fred Couples completed a round of 7-under 65 to jump into fourth place two shots further back.