He is the son of Robert Hartshorne. He started composing for Thomas & Friends with his father during Day of the Diesels and worked on all specials since. He was the sole composer for The Adventure Begins, the first special he composed without working with his father. For this special, he rearranged a number of the original themes by Mike O'Donnell and Junior Campbell. He and his father often work on the music for various documentaries hosted by Andrew Marr, including his most recently released documentary series, "Andrew Marr's History of the World".