Returning to England during the autumn of 1616 he was one of twenty-six personages—and the only one of the number whose father was not a nobleman—who were made knights of the Bath in November of that year on the occasion of Charles being created prince of Wales.

After feebly objecting to more than one of the proposals, he was at last married in 1620 to Martha, eldest daughter of Lionel Cranfield, 1st Earl of Middlesex, who eventually became Lord High Treasurer of England.

Only once does he appear to have come forward to take part in the conflicts of the turbulent times, when he spoke in the House of Lords in June 1641 on the bill for depriving the bishops of their seats in parliament.

When Charles I issued the famous declaration and profession in June 1642, Monmouth's name appears among the signatures, but from this time he retired from all political life, and henceforth till his death he was busily engaged in translating various works from the Italian and French, and letting the world go by him as if he had no interest in its concerns.

The truth is that he had inherited none of the immense physical vigour and energy of his father and grandfather, and if he had any ambition there is no evidence to show that his abilities were at all more than respectable.

In 1658, Monmouth translated Paolo Paruta's Istoria Veneziana (The History of Venice) from Italian into English, the text was subsequently published in London in the same year.