Mann's ownership of Linton gave him electoral interest at Maidstone. At the 1774 general election he contested Maidstone, having deferred a planned journey abroad for his wife's health. He topped the poll and was returned as Member of Parliament for the seat. In 1775 his uncle made over to him the family estate at Bourne, in return for an annuity. He did go abroad and after visiting France, Tuscany, and Austria, returned to England in November 1778. From then on he travelled to his uncle in Florence nearly every summer. At the 1780 general election he was again returned for Maidstone at the head of the poll. He joined Brooks in 1780, and was a member of the St. Alban's Tavern group of country gentlemen who tried to reconcile Fox and Pitt. He did not stand in the 1784 general election.