WASHINGTON (AP) _ J. Roger Mentz, a key figure in the Reagan administration's successful efforts to overhaul the tax law, announced Monday that he was leaving the administration to join a Washington law firm.

Mentz, assistant Treasury secretary for tax policy, will become a partner in the firm of Cadwalader, Wickersham & Taft on Aug. 1.

Treasury Secretary James A. Baker III praised Mentz for his role in helping to push through Congress the major revision of the tax law which took effect on Jan. 1.

''Roger's commitment and expertise helped us overcome many of the obstacles and bring about true and lasting tax reform on behalf of the American people,'' Baker said in a statement.

Before joining the Treasury Department in April 1985, Mentz practiced law in New York and taught at New York University Law School.