Being a younger son, he began a military career when aged 15 in 1793 as a lieutenant in the Strathspey Fencibles. After time in other regiments, he was commissioned a lieutenant colonel in the Third Argyllshire Fencibles in 1799 and served with them as part of the Gibraltar garrison in 1800 and 1801. He received a commission as a full colonel in the British Army in 1809 on appointment as Lord Lieutenant of Inverness-shire. Meanwhile, 'Colonel Grant', as he was known, entered Parliament and followed a political path.