William Ponsonby, 1st Baron de Mauley

William Francis Spencer Ponsonby, 1st Baron de Mauley (31 July 1787 – 16 May 1855) was an English Whig and Liberal politician who sat in the House of Commons between 1826 and 1837. He was raised to the Peerage in 1838.

Ponsonby was the youngest child of 3rd Earl of Bessborough and his wife Henrietta Ponsonby, Countess of Bessborough.

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. In 1838 he was created Baron de Mauley. Whilst an MP for Poole, Ponsonby and Benjamin Lester opened Poole's first public library in 1830.

When the marriage of his sister Lady Caroline Lamb to William Lamb, 2nd Viscount Melbourne, began to break up, he strongly supported Caroline. Unfortunately he was not noted for tact or intelligence—Melbourne's sister Emily Lamb, Countess Cowper, described him as universally regarded as "an ass and a jackanapes". Ponsonby reminded Melbourne that the Lamb family were socially parvenu, and that his sister had married beneath her; although true, these remarks were so tactless that Melbourne broke off any further dealings with him.

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