He was the chairman of the San Diego Padres of Major League Baseball (MLB).

[1][2] He was the grandson of Walter O'Malley, who had owned the Brooklyn Dodgers of Major League Baseball (MLB) and relocated them to the West Coast to become the Los Angeles Dodgers,[3] and nephew of Peter O'Malley who inherited the team, along with Seidler's mother.

[10] The team increased spending in an attempt to contend for a championship, acquiring Matt Kemp, Justin Upton, and James Shields.

He authorized the acquisitions and contract extensions to star players, including Xander Bogaerts, Yu Darvish, Manny Machado, Fernando Tatís Jr., Juan Soto, Josh Hader, and Joe Musgrove.

[14] In September 2023, Seidler announced that he had undergone a medical procedure which would prevent him from attending any further games in the 2023 season.