The son of the sculptor Thomas Thornycroft, Hamo studied under his father, at the schools of the Royal Academy, and in Italy, where he was particularly interested in Michelangelo. He established his own reputation as a sculptor in the 1880s, was elected a royal academician in 1888, and was knighted in 1917. Among his works in London are monuments to General Charles George Gordon (1888, formerly Trafalgar Square), Oliver Cromwell (1899, Westminster), Dean John Colet (1902, St. Paul's School, Hammersmith), William Gladstone (1905, Strand), and the architect Richard Norman Shaw (bas-relief, 1914, New Scotland Yard). His brother was Sir John Isaac Thornycroft, a naval engineer.

Closely identified with the New Sculpture movement of the late 19th century, he became a prominent sculptor of architectural reliefs on public buildings (such as the relief on the Institute of Chartered Accountants, London) in a style that blended naturalistic vigour with Renaissance elegance.