He was born on 11 October 1748, the son of Thomas and Mary (née Leverly) Molyneux.
[2] He conformed to the Church of England on 5 March 1769, for which he was rewarded the title Earl of Sefton on 30 November 1771 with which the viscountcy was merged.
[3][4] He was elected to the House of Commons in 1771 and represented the Lancashire constituency as a Whig until 1774.
[3][5] He died in either late December 1794 or January 1795 and was succeeded by his only son, William.
[2][4] He was uncle of Maria Fitzherbert, royal mistress (and secret wife) of King George IV.