In 1686, therefore, he quitted England with the professed intention of accepting a command in the Dutch fleet then about to sail for the West Indies, When he arrived in Holland, however, he presented himself immediately to the Prince of Orange, and first among the British nobility boldly proposed to William an immediate invasion of England.
He pushed his arguments with fiery zeal, urged the disaffection of all classes, the hatred of the Commons, the defection of the Lords, the alarm of the Church, and the wavering loyalty of the army.
William, however, was already informed of these facts, and was not to be hurried.

Mordaunt remained with him till, on the 20th of October, 1688, he sailed for England.

The first commission that King William signed in England was the appointment of Lord Mordaunt as lieutenant colonel of horse, and raising a regiment he rendered good service at Exeter.

As soon as the revolution was completed, and William and Mary ascended the throne, Mordaunt was made a privy councilor and one of the lords of the bedchamber, and in April, 1689, he was made first commissioner of the treasury, and advanced to the dignity of Earl of Monmouth.

In addition to the other offices to which he was appointed he was given the colonelcy of the regiment of horse guards.
His conduct in office showed in brilliant contrast to that of the men with whom he was placed.