A youthful soldier of the Wars of the Three Kingdoms, he like his father declined to claim the title Lord Somerville, but wrote an extensive work on his ancestry, later edited by Walter Scott.
On the outbreak of the First Bishops' War in 1639, the elder Somerville joined the covenanting levies under General Alexander Leslie, and with the rank of major had a leading command at the siege of Edinburgh Castle in 1640.
In 1648 his father, having purchased from a cousin the old family seat at Cambusnethan in Lanarkshire, moved there from The Drum, and arranged for his son's marriage with Martha Bannatyne of Corhouse.
Towards the end of November he returned with his cousin, Major-general Robert Montgomery, who was in command of a body of cavalry that was designed either to operate against, or come to terms with, the Association levies under Colonels Gilbert Ker and Archibald Strachan.
He went to Renfrew, and arrived in time to take part in a concentration of Royalist forces on Ruglen, which was intended to check Cromwell's advance on Hamilton.
[1] Four Cromwellian regiments of cavalry (Lord Kirkcudbright's, Colonel Strachan's, Ker's, and Halkett's), then made a night march on Hamilton, and occupied the town, but, after a sharp encounter, were driven out and dispersed the next morning.
Somerville, after sending a message to Montgomery, spent three days with the laird of Cathcart, till the country was clear, and then returned to Cambusnethan.
The two folio volumes remained unprinted among the family papers until 1815, when they were edited by Sir Walter Scott, and published with notes and corrections (Edinburgh, 2 vols.).