Ponsonby was elected Member of Parliament for Poole in 1826 and held the seat until 1831, when he lost it in a by-election to Lord Ashley. He was then MP for Knaresborough between June and December 1832. At the 1832 UK general election he was elected MP for Dorset and held the seat until 1837. On 10 July 1838 he was created Baron de Mauley, "of Canford in the County of Dorset". Whilst an MP for Poole, Ponsonby and Benjamin Lester opened Poole's first public library in 1830.