Ferguson appears to have entered the Scots brigade in the pay of Holland, probably as a gentleman volunteer, some time during the reign of Charles II. His first commission, that of quartermaster in Colonel Macdonald's battalion of the brigade, was dated 12 June 1677. He became ensign in the battalion in September 1678, and lieutenant in February 1682. His battalion was one of those brought over to England in 1685 at the time of Monmouth's rebellion. He became captain in 1687, and in 1688 landed with William of Orange at Torbay. His regiment, then known as Balfour's, afterwards as Lauder's, was one of those first landed, and soon after despatched from London to Leith under Mackay. The fight at Killiecrankie, where he is said to have been taken prisoner, left him a regimental major; and in March 1690 he was despatched by General Mackay, who described him as 'a resolute, well-affected officer,’ in whose discretion and diligence he had full reliance, at the head of six hundred men, to reduce the western isles, a service he accomplished satisfactorily with the aid of the Glasgow authorities and the co-operation of Captain Pottinger of the Dartmouth frigate (Ferguson, pp. 15–16).