[1][2] Harker played child parts, including Fleance in William Shakespeare's Macbeth, before being apprenticed to his uncle's trade of scenery painting.

While best known for his work at the Lyceum alongside Hawes Craven and William Telbin (1846–1931), he was also responsible for the complete scenery for Richard Wagner's Parsifal at Covent Garden; for A Life of Pleasure performed at the Theatre Royal on Drury Lane in September 1893 and then at the Prince's Theatre, Bristol, later in that same year, as well as for the musical Chu Chin Chow, which ran for a record-breaking five years in London's West End from 1916.

[1] In 1905, Harker had a two-storey, open-plan studio constructed to his specifications on Queen's Row, a narrow street off Walworth Road in London.

[2] Despite the building being Grade II listed in 1989,[4] as an important and rare surviving example of a theatrical scene-painting workshop, the Southwark Council in early 2017 granted permission for the studios to be redeveloped into six luxury flats and an office unit.

Prior to this decision the ground floor had been occupied by the trade counter for Flints Theatrical Chandlers, which sold paints, brushes and all of the other material required for the scenic arts.