Mompesson served as MP for Southampton until the election of November 1701, when he lost his seat.
Mompesson had become involved in engagements to pay some debts incurred by his father; this placed him in an embarrassing situation.
In April 1703 he accepted an appointment as Judge of the Vice Admiralty for Connecticut, Massachusetts, New Hampshire, New Jersey, New York, Rhode Island and Pennsylvania.
In October 1704 Roger Mompesson was appointed as the first chief justice of New Jersey and, with the exception of a few months during the administration of Lord Lovelace, held office until 14 February 1710.
[2] In April 1706 he was appointed Chief Justice of Pennsylvania, but no evidence exists that he ever entered upon his duties there.