Professional Bancorp's co-chairman resigned in mid-July, just days after the conclusion of a bitter proxy battle.
Lynn Poulson, who will remain on the company's board of directors, could not be reached for comment, and the company said it was uncertain why he had resigned. He is a partner in the Johnson & Poulson law firm.
Director Julie Thompson will act as board chairman. She is chief executive of the Los Angeles-based Health Research Association.
The company announced the resignation July 18 and unveiled plans to streamline the board's committee structure and cut directors' compensation by about 50%.
A struggle between Professional officials and a shareholder group based in Cincinnati ended early this month with the resignation of chief executive and chairman Joel W. Kovner and chief operating officer David Rodeffer.