John Cassidy was born at Littlewood, Slane, Co Meath, Ireland, on 1 January 1860. Prior to leaving Ireland, he worked as an apprentice bar tender at the former White Horse Hotel, West Street, Drogheda where he executed a number of local scenes, two of which are in the Drogheda Municipal Art Collection. By the age of 20, he left Drogheda and worked in Dublin where he attended Art School at night and gained a scholarship to study in Milan, Italy.
On arrival in England, he studied at the Manchester School of Art, and lived in the Manchester area for the rest of his life, establishing a studio in Lincoln Grove. In 1887 he was engaged to give demonstrations in modelling from life at the Manchester Jubilee Exhibition, during which he is said to have modelled more than 200 heads. As his reputation spread, his work was exhibited at the Royal Academy, the Royal Hibernian Academy, and frequently in Manchester City Art Gallery.