Mitchell (1-3) pitched a career-long 5.1 innings and held Winthrop to two runs (one earned) on three hits to go along with two strikeouts.
Senior Daniel Seeba came on and pitched the final 1.1 innings to earn his third save of the season, giving up one hit and striking out one. Freshman Brian Boocock also pitched 2.1 scoreless innings in relief of Mitchell.
Winthrop scored the game's first run in the top of the second when the Eagles pushed across a run on a pass ball. The Eagles loaded the bases on a double from Chad Smith, a walk to Brad Kaczka and an infield single from Luke Maldonado.
Upstate struck for three runs in the fifth inning to open up a 3-1 advantage. Catcher Jordan Miller drew a walk to open the inning and moved to second a sacrifice bunt from shortstop Dylan Woods. Back-to-back walks to Tyler Lesch and Brian Thompson loaded the bases to chase Winthrop starting pitcher Zach Sightler. Brock Goodling entered the game and walked Zach Krider on four pitches to force in a run and allow Upstate to tie the game 1-1. Lesch would later come around to score on a wild pitch with the second run of the inning, and Luke Weber would deliver the game-winning RBI with a groundout on the infield.
Winthrop cut the deficit in half with a run in the sixth inning on an RBI single from right fielder John Menken. From that point on, the Upstate relief corps allowed just three hits and only two runners to reach second base, highlighted with a game-ending double play induced by Seeba.
Goodling (1-2) took the loss after allowing three runs while walking three in 0.1 innings of work.
Lesch turned in a 2-for-3 performance to lead Upstate at the plate, while AJ Mackey and Woods each registered one hit.
Upstate returns to action Wednesday at 6 p.m. when the Spartans host College of Charleston from Harley Park.