After going on a Grand Tour, Spencer returned to England where, in 1754, he was too young to stand for Parliament, but in return for James Grimston's election at St Albans, the Grimston family promised to support for him, or his nominee, on the next occasion. In March 1756 Spencer, standing on the Whig interest, but lost in the exceedingly expensive by-election at Bristol where Jarrit Smyth was elected. Nevertheless, Spencer was returned on 9 Dececember for Warwick, mainly on the interest of Lord Brooke. Spencer represented Warwick alongside Henry Archer. On 27 November 1760, Spencer wrote to the Duke of Newcastle, the Prime Minister of Great Britain, stating:
"As I am the representative of the Sunderland family, and as my particular circumstances are such that I cannot receive any favour from the King except a title, I should hope that if his Majesty thinks me worthy of a peerage he will not confer upon me a less dignity than that of a viscount... I hope your Grace has observed that ever since I sat in Parliament I have never failed in supporting the King and his ministers to the best of my ability."
Newcastle replied that the King had received the application, but that there would be no immediate creations. Therefore, he continued canvassing St Albans but toward the end of February 1761, he nominated Viscount Nuneham in his place, likely after being informed of his impending creation. On 3 April 1761, he was created Baron Spencer of Althorp and Viscount Spencer by King George III. Spencer went into Opposition with Newcastle in 1762, and when the Rockingham Government was being formed, Newcastle noted in his lists: "Viscount Spencer to be created an earl" which happened when he was created Viscount Althorp and Earl Spencer on 1 November 1765. Next, he adhered to the Chatham Administration. Lord Spencer later served as High Steward of St Albans in 1772 and Mayor of St Albans in 1779.
Personal life
In 1754, the twenty year-old Spencer met seventeen year-old Margaret Georgiana Poyntz, who was the daughter of the English diplomat and courtier Stephen Poyntz. The two were instantly attracted to one another, but Spencer was uncertain if he would have his family's approval for the match and opted to spend several months travelling until his twenty-first birthday, when their approval would no longer be necessary. On his return they recognised their love had persisted, and shortly after his birthday they married in a secret ceremony on 20 December 1755 at Althorp, the Spencer family seat in Northamptonshire. The ceremony, held in an upper bedroom, occurred during a ball held in Spencer's honour with five hundred guests in attendance. They had five children, three of whom survived infancy:
Spencer died aged 48, in 1783, near Bath. He had suffered from a long illness. He was buried in the family vault at St Mary's Church, Great Brington in Northamptonshire. His widow lived another thirty years before her death on 18 March 1814.