James Holborne led a brigade of infantry in the Parliamentarian army of the Earl of Essex at the first Battle of Newbury in 1643. He was with Sir William Waller's army at the Battle of Cropredy Bridge on 29 June 1644. At Stowe St Mary, near Tavistock, in January 1645, Sydenham House, a large Elizabethan mansion on the banks of the River Lyd, then being garrisoned for the King, was taken by Colonel Holborne. In April that year, Holborne was offered command of a regiment of foot in the New Model Army, but like several prominent Scottish and Presbyterian officers, he declined.