The MLBPA's executive board voted unanimously Tuesday to appoint Clark to replace Michael Weiner, who died Nov. 22 of brain cancer.
Clark was an All-Star in 2001 and played for 15 seasons with Detroit, Arizona, the New York Mets, New York Yankees, Boston and San Diego.
He went to high school in the San Diego area and later played basketball at San Diego State. Clark says he intends to carry out Weiner's vision and "to continue to bring our fraternity together."
Jeremy Guthrie of the Kansas City Royals calls this "a unique time" for the union and says he can't think of a former player who's as prepared, intelligent, powerful and knowledge as Clark to take the job.