Lewis is a police detective on the staff of the Thames Valley Police in Oxford, England, and in Inspector Morse is assistant to the titular Detective Chief Inspector Morse. Morse's given name (Endeavour) was kept secret until the end of the series and thus he is almost universally referred to only by his family name. Similarly, although Lewis's given name of Robert (Robbie) was not kept secret, he was rarely referred to as anything but "Sergeant Lewis" or "Lewis."

Lewis is a loyal, hardworking, honest, and earnest police detective, who is often troubled or frustrated by the erratic, emotional, and egotistical behaviour of his superior, Morse. The background and personality of Lewis - a working class, easygoing family man with a Geordie accent - is frequently contrasted with that of Morse - Oxford educated, RP-accented, lifetime bachelor. Morse frequently uses these differences to insult or demean Lewis, perhaps from Morse's point of view in a playful manner, but Lewis is often not amused by the jabs. In his frustration, Lewis is often more in step with their joint superior Chief Superintendent Strange, himself an evident supporter of Lewis; however, despite a great respect towards Strange, Lewis is always unflinchingly loyal to Morse and follows his lead.

One notable difference between the Lewis from the novels and Whately's portrayal is that Lewis in the novels is an older man in his early sixties. When Whately auditioned for the role and then learned this fact, he considered it unlikely that he would get the part.[citation needed] However, Colin Dexter has consistently stated that the younger Lewis is an improvement on the character he originally created, and that if he could start the novels afresh he would begin with Lewis as he is seen in the television adaptation.[citation needed]

In Inspector Morse, Lewis is often shown following a hunch that Morse criticises, and in the end Lewis is usually proved correct, or at least more correct than Morse. Near the end of the television series, Lewis moves on in his career and takes a promotion. With the end of Inspector Morse and the death of its star, John Thaw, Lewis's adventures had seemed to come to an end.

In the episode "The Dead of Jericho", Lewis tells Morse "..it's also my birthday". The first victim is killed on June 11, and this utterance occurs a few days to a week later. This puts Lewis' birthday sometime around June 13-June 20.

Lewis does not make an appearance in the episode "The Wench is Dead", as he is out of town on an inspector's course.

Lewis's adventures started again with the creation of an initial one-off episode first broadcast in January 2006. In this episode, Lewis returns to Oxford from a two-year stint training police in the British Virgin Islands, following the death of his wife Valerie in an automobile accident. Lewis still must work partly in the shadow of the now-five-years-dead Inspector Morse, who some time prior to his death had worked a case involving one of the murder suspects as a juvenile. In the new series, Lewis gains his own junior, Detective Sergeant James Hathaway (played by Laurence Fox), a Cambridge-educated man who joined the police on giving up training for the priesthood.

The popularity of the one-off episode spurred the continuation of the story into a new series, with three two-hour episodes aired in January and February 2007. A second series of four episodes then aired in February and March 2008.

Last Bus to Woodstock (1975) · Last Seen Wearing (1976) · The Silent World of Nicholas Quinn (1977) · Service of All the Dead (1979) · The Dead of Jericho (1981) · The Riddle of the Third Mile (1983) · The Secret of Annexe 3 (1986) · The Wench is Dead (1989) · The Jewel That Was Ours (1991) · The Way Through the Woods (1992) · The Daughters of Cain (1994) · Death Is Now My Neighbour (1996) · The Remorseful Day (1999)

Morse's Greatest Mystery (1993)