Edward George Geoffrey Smith-Stanley, 14th Earl of Derby (29 March 1799 – 23 October 1869) was a British statesman, three-time Prime Minister of the United Kingdom and, to date, the longest-serving leader of the Conservative Party. He was known before 1834 as Edward Stanley, and from 1834 to 1851 as Lord Stanley. He was scion of one of Britain's oldest, wealthiest and most powerful families. He is one of only four British prime ministers to have three or more separate periods in office. However, his ministries each lasted less than two years and totalled three years and 280 days. Derby abolished slavery, educated Ireland, and reformed Parliament.
Historian Frances Walsh has written that it was Derby:
who educated the party and acted as its strategist to pass the last great Whig measure, the 1867 Reform Act. It was his greatest achievement to create the modern Conservative Party in the framework of the Whig constitution, though it was Disraeli who laid claim to it.
Scholars long ignored his role but in the 21st century rank him highly among all British prime ministers.