The castle was purchased in 1625 by Captain Henry Upton, who had served under the Earl of Essex, and was renamed for his own family. Upton became Member of Parliament for Carrickfergus in 1634, and several of his descendants served as Members for Carrickfergus and for Antrim County. The family supported the protestant William III in the war against the Catholic James II. In 1776, Clotworthy Upton (1721–1785) was created Baron Templetown after serving in the household of Princess Augusta, dowager Princess of Wales. Lord Templetown commissioned Scottish architect Robert Adam to remodel Castle Upton in a castellated style. Although Adam never visited Ireland his scheme was mostly carried out, including alterations to the roof line and the addition of the north wing. Adam also designed a mausoleum and a stable block, comprising a double courtyard to the north-east of the house. This was completed in 1789, after John Upton had succeeded his father as 2nd Baron Templetown. He sat in the British House of Commons from 1802 to 1812 as one of two representatives for Bury St Edmunds, and in 1806 he was created Viscount Templetown. His successor Henry Montagu Upton, 2nd Viscount Templetown (1799–1863), commissioned Edward Blore to remodel the house in 1837.