The 49-year-old Austin is the eighth oldest winner in PGA Tour history and the oldest since Fred Funk won at 50 in 2007.
Austin came into the final round two shots behind Summerhays and Nicholas Thompson, but made up ground with 5-under 67 to finish at 20-under 268. Beckman and Summerhays both birdied No. 18 to match Austin, and Beckman missed a 5-foot putt that would have forced a second playoff hole.
Austin was making his fourth PGA Tour start of the year. He has four career victories.
LPGA TOUR: In Sylvania, Ohio, Beatriz Recari outdueled Paula Creamer in a head-to-head battle to win the Marathon Classic for her second LPGA Tour victory of the year and third overall.
The 26-year-old Recari closed with a 5-under 66 to finish at 17-under 267.
WEB.COM TOUR: In Overland, Park, Kan., Jamie Lovemark won the Midwest Classic for his second Web.com Tour title, holing a 15-foot par putt on the final hole for 5-under 66 and a one-stroke victory.
The 25-year-old Lovemark finished at 18-under 266 at Nicklaus Golf Club.
Aiken’s Matt Hendrix (64) tied for 11th at 271. Augusta’s Scott Parel (67) tied for 50th at 277.