Irish historian of rationalism and European morals whose study of Georgian England became a classic.

Lecky was educated at Kingstown, Armagh, at Cheltenham, and at Trinity College, Dublin. His early works, Religious Tendencies of the Age (1860) and Leaders of Public Opinion in Ireland (1861), published anonymously, were the products of his eclectic reading while a student and had small success. He had been influenced by the theologian Richard Whately and the historian Henry Thomas Buckle; and his next book, the History of Rationalism (1865), while owing something to them, was welcomed by readers made familiar with evolutionary theory by Charles Darwin and the geologist Sir Charles Lyell. Lecky offered a broadly ranging narrative, showing the emergence of modern scientific thought and rational inquiry since the Middle Ages. The impression made by the considerable learning of this work was deepened by the appearance of its companion study, the two-volume History of European Morals (1869), which explored themes initiated in the former work—the declining sense of the miraculous, the aesthetic expressions of religious belief, and the complex relationship of society and morality. Pervading it was a desire to show the “natural causes” underlying the prevalence of certain theological and moral beliefs.

In 1871 he married Elizabeth van Dedem, lady-in-waiting to Queen Sophia of The Netherlands; but, while this involved him in social duties, it did not interrupt research upon his History of England in the Eighteenth Century, which appeared in 8 volumes (12 in the 1892 edition) from 1878 to 1890 to considerable praise. Lecky's claim to impartiality was not inconsistent with a desire to refute the very different views expressed by James Anthony Froude, particularly evident in the sections dealing with Ireland.

In 1892 Lecky declined the chair of modern history at the University of Oxford and entered politics, being elected in 1895 as a Liberal Unionist to represent Dublin University. His political philosophy is best represented by Democracy and Liberty (1896). He feared the advent of socialism as retrogressive and prophesied a new despotism of the state founded on nationalism and a mass franchise. In Parliament he supported ameliorative measures for Ireland but opposed Home Rule. He was made a privy councillor in 1897 and in 1902 received the Order of Merit.