William Pittman Lett was the great-great-grandson of Thomas Lett (1600–1665), a puritan English captain in the New Model Army, who had moved from Worcestershire to Ireland when Oliver Cromwell invaded in 1649 to conquer an alliance of the Irish Catholic Confederation and English Royalists. William's father, Andrews Lett, continued the family's military and Protestant traditions in the British Imperial Army's 26th Cameronian Regiment of Foot, first mustered in 1689 from the Covenanters to fight for William of Orange to prevent any dilution of the Presbyterian faith. Andrews served under Sir John Moore in the Napoleonic Wars, particularly the Spanish Peninsular War. He married Rebecca Lett, a distant cousin from Ballyvergin, County Wexford and William Pittman was born in nearby Duncannon on 12 August 1819.