McCombs was born in Treanmore, Mohill, County Leitrim, Ireland, the elder child of George McCombs, a farmer, and his wife, Kate Rourke. He came to New Zealand with his parents in 1876 as a three-year-old. He was educated at Sydenham School and Christchurch East School. Initially he intended to join the ministry of the Anglican Church but later decided to cease theological studies and give his time to social work instead. He still remained involved in the Anglican Church and was an active member of the Church of England Men's Society and, inspired by his religious beliefs, he became a prohibitionist and would become a leading member of the prohibitionist movement in Canterbury. He believed that the aims of the Christian Socialism in which he believed were better expressed via community and political activities.