Garfield wrote his first book, the pirate novel Jack Holborn, for adult readers but a Constable & Co. editor saw its potential as a children's novel and persuaded him to adapt it for a younger audience. In that form it was published by Constable in 1964. His second book, Devil-in-the-Fog (1966), won the first annual Guardian Prize and was serialised for television, as were several later works. Devil-in-the-Fog was the first of several historical adventure novels, typically set late in the eighteenth century and featuring a character of humble origins (in this case a boy from a family of traveling actors) pushed into the midst of a threatening intrigue. Another was Smith (1967), with the eponymous hero a young pickpocket accepted into a wealthy household; it won the Phoenix Award in 1987. Yet another was Black Jack (1968), in which a young apprentice is forced by accident and his conscience to accompany a murderous criminal.
In 1970, Garfield's work started to move in new directions with The God Beneath the Sea, a re-telling of numerous Greek myths in one narrative, written by Garfield and Edward Blishen and illustrated by Charles Keeping. It won the annual Carnegie Medal for British children's books. Garfield, Blishen, and Keeping collaborated again on a sequel, The Golden Shadow (1973). The Drummer Boy (1970) was another adventure story, but concerned more with a central moral problem, and apparently aimed at somewhat older readers, a trend continued in The Prisoners of September (1975), The Pleasure Garden (1976) and The Confidence Man (1978). The Strange Affair of Adelaide Harris (1972) was a black comedy in which two boys decide to test the plausibility of Romulus and Remus using one of the boys' baby sister. Most notable at the time was a series of linked long short stories about apprentices, published separately between 1976 and 1978, and then as a collection, The Apprentices. The more adult themed books of the mid-1970s met with a mixed reception and Garfield returned to the model of his earlier books with John Diamond, which won a Whitbread Award in 1980, and The December Rose (1986). In 1980 he also wrote an ending for The Mystery of Edwin Drood, unfinished at the 1870 death of Dickens, an author who had been a major influence on Garfield's own style.
Garfield's novels for children all have a historical setting. In the early novels this is mostly the late eighteenth century, from John Diamond on, it is the nineteenth century. But they are not novels about historical events, which are rarely depicted, or social conditions, which provide only the starting point for the personal stories of the characters. In the few novels where Garfield handles actual events, he writes from the limited and subjective viewpoint of his characters.