From 1899 to 1901 he fought in the Second Boer War in South Africa where, as a captain, he served as a company commander in the 1st Battalion of the Border Regiment.

[1][3] In 1901, Sinclair-MacLagan was seconded to the Australian Army and served as adjutant of the New South Wales Scottish Rifles.

In 1910, he was a major and serving with the Yorkshire Regiment when the then Brigadier William Bridges, who knew Sinclair-MacLagan from his time in Australia, offered him a position as a drill instructor at the newly established Royal Military College at Duntroon.

[1][4] Upon the outbreak of the First World War in August 1914, Bridges was instructed to form the Australian Imperial Force (AIF) for service overseas.

[1] On reaching the high ground at Plugge's Plateau, he quickly realised that his brigade had been landed in the wrong position.

Making the best of a confusing situation, he directed his forces to secure Baby 700, a prominent feature overlooking the ANZAC positions.

[12] He returned to the Western Front in July 1917 when the commander of the 4th Division, Major General William Holmes, was killed shortly after the Battle of Messines.

Its next major engagement was in March 1918 when it was rushed to the Somme sector to counter the German spring offensive.

[1] After the war, Sinclair-MacLagan's rank of major general was made substantive in January 1919,[15] in lieu of the knighthood that other divisional commanders of the AIF received.

[18][19] Sinclair-MacLagan returned to duty with the British Army and served as commander of the 51st Highland Division before retiring in 1925.