MacCallum became a doctor. He became an assistant surgeon for the Royal Sick Children Hospital in Glasgow.[5]
He was a conscientious objector in the First World War. His medical training meant he was assigned a post in the army. At the time he was an Executive Tuberculosis officer for the County of Argyll. He had applied for his conscientious objector status but it had not come through. He was supposed to report to Stirling Castle for his conscription on 30 May 1916 and his non attendance found him in front of a judge in Oban Sheriff Court. MacCallum plead not guilty in view of his conscientious objection but the Sheriff found him guilty and gave him a choice of week's imprisonment or a £2 fine. The headline in The Scotsman "Fined as an absentee" indicated that MacCallum opted for the fine.[6]
