Norman Reginald McMahon was born on 24 January 1866 and was the youngest son of General Sir Thomas Westropp McMahon, 3rd Baronet. McMahon attended Eton College and the Royal Military College Sandhurst from which he was commissioned as a lieutenant in the Royal Fusiliers (City of London Regiment) on 23 May 1885. He served on the Burma expedition of 1886–87 and was appointed adjutant on 28 February 1890. He relinquished his appointment after two years and was promoted to captain on 27 November 1896.