He was the eldest son of Sir Reginald Mohun, 1st Baronet, and was educated at Exeter College, Oxford, graduating in 1608, and joining the Middle Temple.
During 1626 and 1627, he was a member of a number of several commissions in the South-West, including one which inquired into the conduct of Sir John Eliot as Vice Admiral of Devon.
Further, the campaigning was so extreme by the standards of the time that a Commons committee was set up to investigate, and Mohun was summoned to appear at the Bar of the House.
However, Bagg had been trying for months to persuade the King to raise Mohun to the peerage, and his efforts now bore fruit (which not only demonstrated the King's endorsement of the efforts of his supporters in the election, but also put Mohun outside the immediate jurisdiction of the Commons as they could not arrest a member of the House of Lords).
The case rumbled on for some years before the King was able to suppress it and fine Mohun for "undue inquiries into his majesty's debts".