Mohun was born in the year of Charles I’s execution. His family, which had been royalist, chose to submit to the times and reached an accommodation with the Protectorate but was later successful in reasserting its position after the Restoration. Little is known of Mohun’s early life or education, though he seems to have travelled overseas before attaining his majority. He was still abroad at the time of his father’s death but presumably returned soon after as he was noted as being on the point of going to sea with James, duke of York. His imminent departure was the occasion of an order from the king to the dowager baroness requiring her to release into the new lord’s custody the deeds of his estate in accordance with a chancery decree, and in spite of the express desire of the 2nd Baron in his will that his wife retain control of the estates for a year after his heir attained his majority.
With the barony Mohun inherited a substantial estate in Devon and Cornwall, but it was heavily indebted and encumbered by bequests.
Mohun took his seat in the House on 16 Nov. 1670, after which he proceeded to attend on 67 per cent of all sitting days in the session. He was named to 21 committees besides the sessional committees, including that examining the ‘assaulting, wounding and robbing’ of the lord steward, James Butler, duke of Ormond [I] (then sitting in the House as earl of Brecknock), the committee for the bill to prevent the growth of popery and the committee to take accounts of money given to indigent officers. On 22 Apr. he was added to the committee for the Journal. Mohun soon came to be identified with George Villiers, 2nd duke of Buckingham, whose father had secured the peerage for Mohun’s grandfather, and Anthony Ashley Cooper, Baron Ashley (later earl of Shaftesbury), an associate from the West Country.
Such influential friends did not prevent Mohun’s family from continuing to experience the tribulations of indebtedness. On 14 Jan. 1671 Mohun’s mother submitted a petition to the House of Lords complaining of breach of privilege in the arrest of her domestic servant, Eleanor Burford. Lady Mohun’s petition brought to the fore the issue of the ‘privilege of peeresses’, which the committee for privileges was ordered to consider. The Lords later decided that noblewomen and the widows of peers should enjoy privilege of Parliament.
Mohun achieved further redress when his case was heard by the council in November.
Alongside such activities, Mohun continued to seek a resolution to his financial woes. He looked to marriage as a solution and in April 1673 began courting Lady Philippa Annesley.
Mohun returned to the House for the brief four-day session that convened towards the end of October 1673, of which he attended three days. He took his seat once more at the opening of the ensuing session on 7 Jan. 1674, after which he was present on each of the session’s 38 sitting days and was named to a total of six committees, including the sessional committees. Mohun then attended all bar one of the sittings of the House between 13 Apr. and 9 June 1675. Again he was named to a number of committees, but from the outset Mohun’s identification with the opposition was firmly apparent. On 13 Apr. he joined those registering their dissent at the resolution to thank the king for his speech. The following day the House took notice of an ill-tempered exchange between Mohun and John Mordaunt, Viscount Mordaunt, following which Mordaunt was obliged to apologize for his conduct. Both peers were charged not to allow their conflict to continue beyond the floor of the House. Later that month Mohun was to the fore in managing the opposition to the non-resisting test. Between 21 and 30 Apr. Mohun subscribed a series of protests against the passage of the bill, which those opposed to the measure considered an ‘invasion of the liberties and privileges of the peerage’ as well as being ‘destructive of the freedom which they enjoy as members of Parliament.’
Following on from his opposition to the non-resisting test, Mohun also proved to be a central participant in the dispute caused by the case of Sherley v. Fagg. Mohun intervened personally to prevent the serjeant at arms from serving a warrant on Sherley in the Lords’ lobby. He then confiscated the document and presented it to the Lords for their consideration. The Commons complained about Mohun’s actions and demanded that the Lords censure him for his behaviour. Although the Lords agreed to consider the Sherley affair, the House insisted that Mohun had done nothing wrong. Perhaps significantly, Anglesey and Christopher Monck, 2nd duke of Albemarle, registered their dissents at this resolution.
Following the prorogation Mohun joined Shaftesbury in campaigning on behalf of Thomas Moore‡ against John Digby, styled Lord Digby (later 3rd earl of Bristol) in the Dorset by-election. Shaftesbury had initially offered to back Digby but then reneged on the agreement, which helped to contribute to a particularly fractious election. The result was a landslide victory for Digby, but he was later sued for scandalum magnatum and ordered to pay £1,000 to Shaftesbury, in part as a result of compelling evidence provided by Mohun.
Mohun returned to the House at the opening of the new session on 13 Oct. 1675, after which he was present on 95 per cent of all sitting days. He was nominated to the three sessional committees and to an additional five select committees, including that considering the explanatory bill for the act against popish recusants and the committee for discovery of the publishers of the Letter from a person of quality, which was ordered to be burnt. The early stages of the session were again dominated by the dispute between Lords and Commons over Sherley v. Fagg. Mohun continued to play an active role in the affair, and on 19 Oct. he presented Sherley’s petition to the House. The Lords later resolved that Sherley, the king’s physician, was entitled to privilege as he had a suit pending before them.
Family disagreements emerged once more in November 1675 when the House was informed of differences between Mohun and his mother over finances. Shaftesbury, Arthur Capell, earl of Essex, and Denzil Holles, Baron Holles, were appointed to arbitrate but the dispute was never resolved.
Mohun was not one of those summoned to act as a tryer for the trial of Charles Cornwallis, 3rd Baron Cornwallis, in July 1676 but it was noted that throughout the proceedings he, Shaftesbury and Philip Wharton, 4th Baron Wharton, ‘sat in the king’s sight, whispering together all the time.’
The following month, on 17 Nov. Mohun again found himself embroiled in a violent altercation. The occasion was a duel against an Irish officer in the French army, John Power, in which Mohun acted as second alongside his friend William Cavendish, styled Lord Cavendish (later duke of Devonshire).
Medical opinion was divided about the severity of Mohun’s injuries. Within a few days of the encounter he was believed to be making a good recovery, but his case was uncertain and over the next few months his condition remained in the balance. In December, with his prospects for recovery fading, it was noted that he had very generously signed a declaration seeking to clear Cavendish of any involvement in the second duel. Cavendish on the other hand was heavily criticized for showing scant regard for his companion’s condition.
Mohun defied his doctors’ predictions and rallied in February.
Mohun was buried at St Martin-in-the-Fields on 3 Oct. 1677. He was succeeded in the peerage by his six-month-old son, Charles Mohun, as 4th Baron Mohun.
