Lecky then devoted himself to the chief work of his life, A History of England during the Eighteenth Century, vols. i. and ii. of which appeared in 1878, and vols. vii. and viii. (completing the work) in 1890. His object was "to disengage from the great mass of facts those which relate to the permanent forces of the nation, or which indicate some of the more enduring features of national life". In carrying out this task, Lecky displays many of the qualities of a great historian. The work is lucid in style, extensive in its use of source material, and, above all, impartial throughout. These qualities are particularly valuable in the chapters dealing with the history of Ireland, and in the "cabinet" edition of 1892, in 12 volumes (frequently reprinted), this part of the work is separated from the rest, and occupies five volumes under the title of A History of Ireland in the Eighteenth Century.
A volume of Poems (1891) was less successful. In 1896 he published two volumes entitled Democracy and Liberty, in which he considered, with special reference to Britain, France and America, some of the tendencies of modern democracies. The pessimistic conclusions at which he arrived provoked criticism both in Britain and America, which was renewed when he published in a new edition (1899) an elaborate and very depreciatory estimate of Gladstone, then recently dead. This work, though essentially different from the author's purely historical writings, has many of their merits, though it was inevitable that other minds should take a different view of the evidence.
In The Map of Life (1900) he discussed in a popular style some of the ethical problems which arise in everyday life. In 1903 he published a revised and greatly enlarged edition of Leaders of Public Opinion in Ireland, in two volumes, from which the essay on Swift was omitted and that on O'Connell was expanded into a complete biography of the great advocate of repeal of the Union. Though always a keen sympathizer with the Irish people in their misfortunes and aspirations, and though he had criticized severely the methods by which the Act of Union was passed, Lecky, who grew up as a moderate Liberal, was from the first strenuously opposed to Gladstone's policy of Home Rule, and in 1895 he was returned to parliament as Unionist member for Dublin University in a by-election. In 1897 he was made a privy councillor, and among the coronation honours in 1902 he was nominated an original member of the new Order of Merit.
In 1904, money for a memorial was raised by subscription and a statue by John Goscombe was erected in Trinity College, Dublin.
A volume of Lecky's Historical and Political Essays was published posthumously (London, 1908).
The Lecky Chair of History at Trinity College, Dublin was endowed by his widow in 1913.