Knight was born in Nottingham, England, and first studied at the Nottingham School of Art under Wilson Foster. He spent a year in Paris at the Académie Julian, studying under Jean-Paul Lawrence and Benjamin Constant. In 1896, Knight exhibited at the Royal Academy, London and then at the International Society.
While at the Nottingham School, he met Laura Johnson, whom he married in 1903. Following their marriage, the Knights spent several years in Holland studying the Dutch masters. The Knights traveled to Baltimore in 1926, where Harold had commissions for portraits of Johns Hopkins faculty. Laura sketched and painted patients and nursing staff at The Johns Hopkins Hospital.
Harold and Laura Knight were both elected to the Royal Academy, Laura being the first woman to achieve this honor. Harold Knight had a long and productive career. His large body of works includes portraiture, genre, and landscape painting. His work is included in the collections of the Tate, the National Portrait Gallery, and the Royal Academy, London; in the collection of the National Gallery of Australia; and in various other galleries and museums.