Jenkins began the round a stroke off Irwin's pace but matched the best score of the day with a 5-under-par 67 that included six birdies and a bogey. His 54-hole total of 16-under 200 was three shots better than Bruce Fleisher.
It was the fourth Champion Tour victory for the 55-year-old Jenkins, who won the Canada Senior Open Championship in 2000 and 2002. His only other victory in the United States came at the 1999 Bell Atlantic Classic in Pennsylvania.
The winningest player in tour history, Irwin remained without a title this season, shooting a 1-over 73 that left him in a tie for third with Jim Colbert at 11-under 205.