Brock received his first book commission at the age of 20 in 1890. He became very successful, and illustrated books for authors such as Jonathan Swift, William Thackeray, Jane Austen, Charles Dickens, and George Eliot. Brock also contributed pieces to several magazines such as "The Quiver", "The Strand", and "Pearsons". He used the Cambridge college libraries for his "picture research." In illustration Brock is best known for his line work, initially working in the tradition of Hugh Thomson, but he was also a skilled colourist. As a painter he received plaudits for his realism and vibrancy he created in his work. Only a small quantity of his paintings have been located which is why their prices have been so high.