Spong is a graduate of Leicester College of Art, like John T. Kenney and Clarence Reginald Dalby, who were among the earlier illustrators of the Railway Series books. He remained the illustrator throughout the run of Christopher Awdry's 14 books, illustrating more titles in the series than any other artist.
Spong was one of the first illustrators to break Wilbert's golden rule about engines having no faces outside of Sodor – he did this in book 32 of the series, Toby, Trucks and Trouble – depicting Toby and his brother with faces whilst outside Sodor; in book 35, Thomas and the Great Railway Show – depicting National Collection engines such as Mallard, Duchess of Hamilton, Green Arrow and Iron Duke (complete with a mustache) with faces; and in book 38, Wilbert the Forest Engine – depicting Sixteen with a face when on his railway outside of Sodor. He also illustrated pop-up books including Henry and the Elephant from Troublesome Engines.
Spong illustrated pop-up books and the spin-offs, Thomas's Christmas Party, Thomas and the Evil Diesel, Thomas and the Missing Christmas Tree, and also The Rev. W. Awdry's extended rewrite of Thomas Comes to Breakfast. Some of his character illustrations were used to depict characters in The Island of Sodor: Its People, History and Railways