William Kempner

William McCullough Kempner

• Bats Right, Throws Right
• Height 6' 0", Weight 222 lb.
• School Gonzaga University
• High School Valley Christian High School (San Jose)
• Debut May 5, 2026

• Born June 18, 2001 in San Jose, CA USA

BR page

Biographical information

Pitcher William Kempner was drafted by the San Francisco Giants in the 3rd round of the 2022 amateur draft, out of Gonzaga University. He spent his first professional season in 2022 with the ACL Giants Black and with his hometown minor league team, the San Jose Giants, going a combined 1-0, 5.00 in 5 games. In 2023, he played with three different teams, starting back with San Jose in the California League, then earning successive promotions to the Eugene Emeralds of the Northwest League and the Richmond Flying Squirrels of the Eastern League, reaching AA in a little over a year. His most memorable game came with Eugene on July 4th as he was the third of four pitchers to combine to throw a no-hitter against the Tri-City Dust Devils that day. Hayden Birdsong pitched the first five innings, succeeded by Mat Olsen for the next two. The Emeralds had just taken a 1-0 lead in the 7th when he came in to pitch the top of the 8th, and he retired the side in order, picking up two strikeouts, before Sam Delaplane handled the 9th. Overall, in 38 games between the three teams, including 5 starts, he went 4-5, 3.73 with 1 save and 73 strikeouts in 62 2/3 innings.

He would normally have returned to Richmond for the start of the 2024 season, but instead went on the injured list coming out of spring training. He ended up missing the entire season, although he was able to pitch in the Arizona Fall League. The following January, he was traded to the Miami Marlins in return for international bonus pool money. He pitched at three different levels in 2025, with 19 games for the Beloit Sky Carp of the Midwest League, 16 for the Pensacola Blue Wahoos of the Southern League, and 13 for the Jacksonville Jumbo Shrimp of the International League, his first time reaching AAA. The common thread was that he pitched well in relief at all three stops, finishing with a combined record of 7-3, 2.26 and 7 saves in 48 games and 67 2/3 innings. He struck out 95 batters while walking 40.

He started the 2026 season with Jacksonville. He was 0-3 with an ERA of 6.46 after 12 games, although he had also picked up 3 saves. On May 5th, he was called up to Miami and made his debut the same day, pitching a scoreless inning of relief in a 9-7 loss to the Baltimore Orioles.

Related Sites

• BR Minors page
• Gonzaga University bio