The former Harlequins prop, who won 114 caps for England between 1990 and 2004, joins fellow Rugby Football Union representatives Francis Baron, Rob Andrew and John Spencer on the Professional Game Board.
The board, which will be chaired by RFU management board chairman Martyn Thomas, also contains four Premier Rugby Limited representatives - Tom Walkinshaw, Mark McCafferty and Peter Wheeler, with the fourth position still to be filled.
The PGB will begin work next summer, having been established as part of the new long-term agreement signed last month between the RFU and PRL.
Leonard said: "I am delighted to be joining the PGB.
"I am a huge supporter of the elite game, and I hope I can bring the substantial playing experience I enjoyed to the meetings, which are aimed at ensuring the elite game is the best it can be at club and international level."