Stewart, 57, replaces Kevin Towers, who was fired earlier this month.
Stewart won 168 games during his 16-year career, which included four 20-win seasons, and pitched for five teams, most notably the Oakland Athletics. He later became a player agent who represented Los Angeles Dodgers players Matt Kemp and Chad Billingsley as well as Houston Astros designated hitter Chris Carter.
Named World Series MVP after Oakland’s four-game sweep of the San Francisco Giants in 1989, Stewart will be reunited with former A's manager Tony La Russa, who is now Arizona’s chief baseball officer.
The Diamondbacks also hired former minor leaguer De Jon Watson, who has served as a scout and scouting director with with the Marlins, Reds, Indians and Dodgers. He'll now be taking a role in player development for Arizona.
"I've been fortunate enough to enjoy success as a player and coach and really enjoyed my time representing players, but this is the perfect challenge for me," Stewart said in a statement. "To try and put together the pieces of a championship team is something I can't wait to begin, and I could not be more excited about doing it with De Jon and alongside a Hall of Famer who I respect greatly in Tony.”
The Diamondbacks will finish the 2014 season with the worst record in the National League.
- Scooby Axson