McLaren had schooled at Harrow and studied at Sandhurst before joining his regiment, the 13th Hussars in 1880. He was posted to India, where he served as regimental adjutant and later aide de camp to General Baker Russell. In South Africa he was gravely wounded at the Siege of Mafeking in March 1900, and taken prisoner by the Boers. He was created a Companion of the Distinguished Service Order (DSO) in November 1900.