Mountjoy was diagnosed with lung cancer in 1993, having been a smoker for many years. That year, at the age of 50, he defeated Alain Robidoux 10–6 in the first round of the World Championship, only weeks before an operation to remove his left lung. This was his last appearance in the final stages of the championship and for fifteen years he was the last player aged over 50 to appear at The Crucible. He survived the cancer and continued to play snooker until 1997, after which he concentrated on snooker coaching but did appear in the 2002 World Snooker Championship.