Fergusson was educated at Eton College and the Royal Military College, Sandhurst. From the latter, he was commissioned as a second lieutenant into the Black Watch on 27 August 1931. He was promoted to lieutenant on 27 August 1934. He served with the 2nd Battalion of his regiment in the British Mandate of Palestine during the Arab revolt and later became aide-de-camp (ADC) to Major General Archibald Wavell, then General Officer Commanding of the 2nd Infantry Division in England, on 11 March 1937. In October 1937, he was on secondment to the Green Howards. Fergusson was promoted to captain on 27 August 1939, only a few days before the outbreak of the Second World War.