He finished the tournament at 17-under 199 and became the 16th player to have a winning debut on the Champions Tour.
Mediate's 4-foot birdie putt came after Tom Pernice Jr. missed a 5-footer at No. 18 that could have forced a playoff.
Mediate started the day with a three-shot lead, lost it by the 14th hole, but regained it when Pernice bogeyed the par-4 17th from a greenside bunker. Mediate protected that lead with a birdie on the par-5 closing hole.
"I'm ecstatic," Mediate said. "I have been saying all week it's never easy. It shouldn't be easy. I was prepared to make that putt (at No. 18), but I didn't have to."
Pernice, who had a final-round 70, finished tied with Bernhard Langer (68) for second place.
Pernice was the last player to win his Champions Tour debut three years ago, but is still searching for his second title.
"I played better than any other day as far as ball-striking, but I just couldn't make enough putts," Pernice said. "Rocco played well; he deserved to win. He battled down the stretch."
Langer was within a shot of Mediate on the back nine, but also couldn't make any key putts. It marked Langer's fourth top-three finish in this event in the past five years. Langer won here in 2010.
"The course played really fast and firm," Langer said. "I actually hit the ball too far a number of times. But two three-putts really hurt."
Mediate won six PGA Tour titles and also lost a memorable playoff to Tiger Woods in the 2008 U.S. Open at Torrey Pines. Mediate shot a course-record 61 in the second round here Saturday to help him match Arnold Palmer, Jack Nicklaus and Gary Player, among others, of winning his inaugural Champions start.
"This means as much to me as anything I've done," Mediate said.
Mark O'Meara (69) and defending champion Corey Pavin (69) were both six shots off the lead.
More Sports Headlines
Snohomish mixed martial artist returns to her first love Mariners beat Twins 4-3 in 11 innings Charles scores 29 points, Liberty beat Storm 78-62 Park wins Women's British Open, captures 4th different major Perry wins 2nd straight Champions Tour's 3M Championship Merritt shoots 67 to win Quicken Loans National by 3 shots Triple Crown winner American Pharoah wins $1.75 million Haskell Invitational Bills' Ryan names Incognito starting left guard
Our new comment system is not supported in IE 7. Please upgrade your browser here.