The first entry of his name in existing War Office records is 24 February 1704, when he was appointed cornet in Captain Henry Hunt's troop of Colonel George Carpenter's dragoons.
It fought with distinction under King William III in the Irish and Flanders campaigns; part of it was in the Cadiz expedition in 1702; and it also served in Spain in 1707–8, and suffered heavily at the battle of Almanza, after which it was sent home to be reformed.
There he is said to have received letters, signed by George I, directing him in case of disorder 'to burn, shoot, or destroy without asking questions, for which and all that he should do contrary to the law in execution of these orders he thereby previously indemnified him.'
Upon learning of the landing of Charles Edward Stuart and the rallying of his Jacobite supporters, Guest was ordered to send ships to bring Sir John Cope and his troops from Inverness in the north, to reinforce Edinburgh.
Gardiner's Dragoons had fallen back before the rebels before joining with Hamilton's regiment near Edinburgh at Guest's suggestion to await Cope's reinforcements.
According to some he was offered and refused a bribe of £200,000 to surrender the castle; others including Robert Chambers in his 'Memorials of Edinburgh,’ who bases his assertions on 'information received from a member of the Preston family,’ declare that Guest was a Jacobite at heart; that at the council of war held on the arrival of the fugitives from the battle of Prestonpans he proposed to surrender, as the garrison was too weak to defend the place if attacked; and that this proposal was successfully opposed by George Preston, who remained in the castle as a volunteer, and according to this version was the real defender of the stronghold.
The castle was successfully held by General Guest and his garrison during the time Edinburgh was occupied by the rebels, the defenders cannonading Prince Charles's followers at the review preceding their march into England.
Preston, a veteran of eighty-seven, who, it is said, was wheeled round the guards and sentries in a chair every two hours during the hottest part of the blockade, went to his Scottish home unrewarded.