Molyneux was descended from Sir Thomas Molyneux, chancellor of the Exchequer in Ireland under Queen Elizabeth. His father, a distinguished mathematician with substantial estates in Armagh, Kildare, and Roscommon, died when he was young, leaving him to the care of his uncle, Dr. Thomas Molyneux, a physician. While at Trinity College he began to devote himself to astronomy, which became his life-long study. In 1712 he visited the Duke of Marlborough at Antwerp, whence he proceeded with the Duke’s recommendation to Hanover, where he was received with great favour. A few months before the death of Queen Anne he was sent by that court over to England on a secret mission.
I must tell you a story of Molyneux: the other day at the Prince’s levee, he took Mr. Edgcumbe aside, and asked, with an air of seriousness ‘what did the Czar of Muscovy, when he disinherited his son, do with his secretary?’ To which Edgcumbe answered ‘he was sewed up in a football, and tossed over the water’.
Works, ed. Elrington Ball, ix. 395.
At this time Molyneux told his kinsman, the first Lord Egmont, that when everyone was forced to choose between the King’s and the Prince’s courts ‘he had computed what every person concerned lost or gained by the party they chose, and that he found for £20 advantage the Prince’s court abandoned or stayed with him’.
At George II’s accession Molyneux, having failed to secure a seat in the previous Parliament till 1726, was appointed to the Admiralty board. Returned for Exeter, which he had unsuccessfully contested in 1722, he was seized with a fit in the House of Commons, dying a few days later, 13 Apr. 1728, aged 38. Soon after his death his widow married a Swiss surgeon, who won an action for defamation on a charge of having killed him by administering opium to him in his last illness with her connivance.