A by-election was held for the New South Wales Legislative Assembly seat of Mosman on Saturday, 29 July 1972. It was triggered by the retirement of the former Leader of the New South Wales Liberal Party and Minister of the Crown, Pat Morton. The seat was subsequently won by David Arblaster (Liberal). Despite Mosman being a safe Liberal seat, the Labor Party fielded a candidate. The Liberal primary vote fell 11.1% compared to the previous year's general election. The same four parties contested both.