William Henry Lynn (1829–1915) was an Irish-born architect with a practice in Belfast and the north of England. He is noted for his Ruskinian Venetian Gothic public buildings, which include Chester Town Hall (completed 1869) and Barrow-in-Furness Town Hall (completed 1886).
In 1846 Lynn was articled to Sir Charles Lanyon in Belfast; under Lanyon he prepared the drawings for the original building housing Queens College, Belfast. He and Lanyon formed a partnership in 1854; in 1860, with Charles' son John Lanyon as junior partner, they incorporated as Lanyon, Lynn and Lanyon. The partnership dissolved in 1872, when Lynn struck out on his own.
For their first joint projects (1855), Lynn and the elder Lanyon produced bank buildings at Newtownards, County Down, and at Dungannon, County Tyrone, which are two of the earliest Irish examples of the Venetian Gothic style that was being championed by John Ruskin. In Belfast the firm produced urbane Italianate commercial structures, in Dublin, the Church of St Andrew (1860) and the Unitarian Church, St Stephen's Green was "justly described as the best example extant of a modern Gothic church on a narrow street frontage, the treatment being quite original and altogether admirable". In Jordanstown, Co. Antrim, they designed the Romanesque Revival Church of St Patrick (1865–8) and, in England, the Chester Town Hall, following a public competition (1863–9), should be mentioned. In the 1860s a second Shane's Castle was designed with Charles Lanyon for 1st Lord O'Neill (burned 1922), and in 1870 Castle Leslie was designed for Sir John Leslie, 1st Baronet.