Which British prime minister was in office at the start of Victoria's reign?

Lord Melbourne was the British prime minister in office when Queen Victoria began her reign in 1837. William Lamb, 2nd Viscount Melbourne, led the Whig government when King William IV died on 20 June 1837 and Victoria succeeded to the throne.

Melbourne had already become prime minister in 1834, briefly lost office later that year, and returned in 1835. He remained in office until 1841, so he guided the young queen through the opening years of her reign. Victoria was only eighteen when she became monarch, and Melbourne became an important political adviser and personal mentor.

The other options were also prominent Victorian prime ministers, but they served later. Sir Robert Peel followed Melbourne in 1841. Benjamin Disraeli held office during two later periods, including the years surrounding Victoria’s assumption of the title Empress of India. William Gladstone served four separate premierships, beginning in 1868.

The phrase “at the start” matters: Melbourne was prime minister on the day Victoria inherited the throne, not merely the first prime minister she appointed after a change of government.