American Lehman fired a four-under-par 68 in persistent rain during the final round to finish at 14-under 274, holding off strong final round charges by German Bernhard Langer (66), Taiwan's Chien Soon-lu (66) and fellow American Fred Couples (65).
"It was a good day. It was a good week. To win makes it that much more special," said Lehman, the first person to repeat in a senior Major championship since Allen Doyle won the U.S. Senior Open in 2005 and 2006.
"But I don't like playing in the rain, I never have ... To me it kind of makes me really uncomfortable. I feel like I lose rhythm of my swing.
"So today was a real test of perseverance ... don't get too aggressive, just play smart shots, shots that I know I can hit and make a par, make another par, maybe sneak a birdie in here and there."
A bogey on the second hole could have easily rattled the 53-year-old former Open champion, but he stayed in control with six birdies over the next 14 holes.
Langer and Chien finished tied for second at 12-under 276, leaving Couples alone in fourth a further shot back.