Mahony made his Wimbledon debut in 1890 exiting in the first round. He reached the semifinal in 1891 and 1892. Mahony spent some time in America in the mid-1890s, before returning to England and finally taking the Wimbledon crown in 1896. In the final he beat Wilfred Baddeley of Great Britain in five sets: 6–2, 6–8, 5–7, 8–6, 6–3. Under the challenger system Mahony was entitled to defend the Wimbledon title in 1897 but this time he was beaten in the Challenge Round in three straight sets by Reginald Doherty. He was the last Scottish-born player to win a grand slam until Andy Murray won the US Open in 2012 and win Wimbledon until Murray won it in 2013. He was recognised as the third and last Irishman to win the Wimbledon singles.