Harris began writing freelance episodes for British television in his mid-twenties. His first sale was to Sydney Newman's 1960 ITV series, Police Surgeon, for which he wrote the final episode, "The Bigger They Are". Though he wrote for the initial runs of The Avengers and The Saint, much of the early 1960s was dominated by his contributions to anthological mystery programmes like The Edgar Wallace Mystery Theatre and situation comedies like Hancock. His attempts at comedies in the early 1960s were largely collaborative efforts with Dennis Spooner. These joint efforts did not establish either writer in the comedy genre, Instead, as their two failed pilots for Comedy Playhouse proved, the duo were really more interested in writing dramatic works. Despite his commercial failures with Spooner, he continued to collaborate with others during his early career...perhaps most successfully in 1966's Adam Adamant Returns!, whose pilot he wrote with Donald Cotton. By the end of the decade, he had contributed individual episodes to no less than twenty series.

From the late 1960s onward, producers began allowing him to write a number of "first episodes", effectively making him co-creator of a number of projects like The Gamblers and Life and Death of Penelope. Despite having turned a number of ideas into initial scripts, however, he only occasionally received on-screen credit as co-creator. This pattern is evident in two of his most recent shows, both adapted from literature. On The Last Detective, he is recognized as having "devised the series for television". On A Touch of Frost, he is not...despite having written the entirety of the programme's first season.

Beginning in about 1971. Harris turned his earlier comedic ambitions towards the stage. The vast majority of his comedic work, even if it later ended up film, derives from his career as a playwright. Throughout the 1970s, a new play of his would be produced almost annually. Though the frequency of his stage work slowed in later decades, his plays continued to debut into the early part of the 21st century.

While the vast majority of his career has been spent as a freelancer, he has been an occasional script editor, with shows like Hazell and Hunter's Walk.

He is an intermittent radio dramatist, and has won the Giles Cooper Award for adapting his teleplay, Is It Something I Said? One of his most successful plays, Stepping Out, has appeared in three different versions, ultimately allowing him the opportunity of musical film adaptation.

A substantial part of Harris's body of work is adaptation. Often, as in examples cited above, this has taken the form of adapting his own work from one medium into another. However, he has also taken a number of literary characters and adapted them into ongoing series. The most long-running of these adaptations are A Touch of Frost and The Last Detective, but he has also converted works including Mark Twain's The Prince and the Pauper into limited-run serials. A third and more minor kind of adaptation has been the conversion of non-English sources into English drama. His play, The Last Laugh, derives from a Japanese work, and his adaptation of a Norwegian source into the dual-language film, Orions belte, won the inaugural Amanda for Best Norwegian Film in 1985.

Because Harris is a contemporary of the late Richard St. John Harris, his writing credits are sometimes erroneously ascribed to the Irish actor.