Llewellyn joined the cast of Red Dwarf in 1989 in series 3 and continues in the role through the present with the twelfth series, in 2017. In May 2015, it was announced that he would, again, be returning to Red Dwarf for the twelfth series. His skills as a physical performer encouraged Rob Grant and Doug Naylor to write him additional characters for the series, namely Jim Reaper ("The Last Day"), The Data Doctor ("Back in the Red"), Human Kryten ("DNA"), Bongo ("Dimension Jump") and Able ("Beyond A Joke"). Llewellyn co-wrote the Red Dwarf Series VII episode "Beyond A Joke" with Doug Naylor.