 He was a Member of Parliament for Helston from 1802 to 1805.In 1805, he was appointed as governor of the Isle of Portland, where he built Pennsylvania Castle and later the sea bathing stone bath known as John Penn's Bath, close to the gardens of the castle.In 1818, still a bachelor at 58, Penn founded the Matrimonial Society, soon renamed the Outinian Society to encourage young men and women to marry.