He eventually reached the rank of Major General and fought with the Parliamentarians during the English Civil War, fighting at Newbury and Taunton.
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.
Holborne was a Major General by 1645, when he was nominated, with the Earl of Leven, and Lord Kirkcudbright, as a deputation from the Convention of Estates, the most powerful party in Scotland at that time, to open negotiations with Oliver Cromwell, whose army was then at Berwick.
On 5 May, Montrose thus begun his long and humiliating captive journey, and on 6 May, Major General Holborne took shelter at Skibo Castle, the home of the dowager Lady Gray.
With this breach of etiquette, the Lady Gray flew into a violent rage, and seizing upon a leg of roasted mutton by the shank she confronted the Major General with "such a notable blow on his head, knocking him of his seat.
Montrose was led down the east coast of Scotland on the long journey toward Edinburgh, where he was met at the town's Watergate and the sentence of hanging, drawing and quartering was pronounced.