Murchison was born at Tarradale House, Muir of Ord, Ross-shire, the son of Kenneth Murchison. His wealthy father died in 1796, when Roderick was four years old, and he was sent to Durham School three years later, and then the Royal Military College, Great Marlow to be trained for the army. In 1808 he landed with Wellesley in Portugal, and was present at the actions of Roliça and Vimeiro in the Peninsular War as an ensign in the 36th Regt of Foot. Subsequently under Sir John Moore, he took part in the retreat to Corunna and the final battle there.