Tewell had fired a 6-under 66 to tie Darcy atop the leaderboard after Darcy carded a 7-under 65. The two men headed to extra holes after finishing the tournament at 15-under-par 201.
Back at the 18th tee at Egypt Valley Golf Club for the first extra hole, the pair both missed the green long and were unable to get up-and-down for par. After the bogeys, they headed to the 10th tee for the second extra hole.
On No. 10, the two found the fairway off the tee and each knocked their second shot on the green. Tewell's birdie try stopped within inches of the cup before he tapped in for par while Darcy also two-putted for par.
The duo returned to the 18th tee for the playoff hole. Darcy hit a wayward tee shot but got a lucky bounce off a cart path and into the fairway. Tewell bombed his drive right down the middle of the fairway.
Darcy found the green some 50 feet from the cup while Tewell dropped his second shot within 13 feet of the hole. Darcy's birdie try came up just short and Tewell ran home his birdie attempt for the win, his seventh victory on the Champions Tour.
Trailing by one stroke after a bogey at 16, Tewell eagled the par-5 17th, while Darcy managed a birdie. Both men parred the final hole to force the playoff.
Defending champion Jay Sigel recovered from a poor second round with a closing 6-under 66 to finish at 4-under-par 212 in a tie for 32nd.