He returned to Dundas in 1852 and joined the practice of Doctor Mitchell. After Mitchell died of cholera in 1854, McMahon took over his practice. He married Julia Ball in 1858.
McMahon resumed his medical career in Dundas in 1852, working alongside Doctor Mitchell. Following Mitchell's passing due to cholera in 1854, McMahon took over the doctor's practice. In 1858, he wed Julia Ball.