Walmsley was born in Dublin into a military background. After a quarrel with his family, he moved to London and worked as a scene painter at the Royal Opera House. In 1788, he returned to Ireland for two years to work at the Crow Street Theatre. The inscription ‘J. Kemble’ may be a reference to the actor John Philip Kemble (1757-1832).

Walmsley regularly exhibited landscapes at the R.A., including many of Wales, Ireland, Scotland and the Isle of Wight. Much of his work was aquatinted and published. At the end of his life he retired to Bath. Examples of his drawings may be found in the British Museum, the V&A, the Ashmolean in Oxford and many other institutions.