Starter Joey Haston pitched six strong innings, but faltered in the seventh, allowing Union City to close a 3-0 deficit to tie the game.
"He had such a quick six, I thought I'd leave him in for the seventh and save my staff," Manager Steve Fowler said of his initial plan to pitch Haston, 1-2 with a 1.61 ERA in the regular season, for only six innings.
But Marion, as they have tended to do during the regular season, won the game with some late-inning drama.
"We had flawless execution in the ninth," Fowler said.
Jacob Martin led off the last frame with a walk and was moved to second on a sacrifice by Adam Enloe. Lucas Cramsey, who ended the season with a 25-game on-base streak, moved Martin to third, allowing Mitch McClay to score Martin on bloop hit. Closer Kyle Medley, with eight saves on the regular season, finished the game with a postseason save by throwing a perfect inning.
Chad Adcock, who pitched the eighth inning against the Greyhouds, earned the win.
"It's pretty big," the manager said of the first-year team's first playoff win.
Marion now advances to the second round of the playoffs, traveling to Tradewater (35-15) Tuesday for the first game of a best-of-three series against the league's best team. The second game of the series will be played at Gordon B. Guess Field in Marion. Game 3, if necessary, will return to Riverside Park in Dawson Springs.
"It's going to be an all-out war," Fowler said of the series. "They want us!"
The winner of the Marion-Tradewater series will advance to the championship to face the winner of the Owensboro-Fulton series, which also gets under way Tuesday. Owensboro (27-20) beat Farmington (8-38) Monday 6-2 to advance to the second round.
Marion beat Union City Monday for the seventh time this season. The Greyhounds are the only team against whom the Bobcats had a winning record in the regular season, 6-4.
Fowler's club faired almost as well against his old team, the Pirates. The Bobcats were 5-5 against Tradewater this season, including three consecutive wins against Tradewater to end the season series. Tuesday's playoff game should be a continuation of the hottest rivalry in the KIT League.
"We're either going to wreck their season, or ours is going to come to an end," Fowler said after Monday's playoff win.
Fowler has not yet decided on his starter for Game 1 of the Tradewater series, but the Pirates have the league's top two winning pitcher in George Hebert (8-0) and Don Pugliese (7-0). The game at Dawson Springs will start at 7:05 p.m.
Game 2 of the Marion-Tradewater series will be at Gordon B. Guess Field in Marion, with a 7:05 p.m., start time.