As a comedian in the 1980s Dennis has appeared on numerous light-entertainment shows and worked originally as a stooge to Russ Abbott on Russ Abbott's Madhouse before forming a successful comedy partnership with fellow impressionist, the late Dustin Gee. Following Gee's death in 1986, he is probably best known as the host of Family Fortunes, a fifteen year run from 1987 to 2002. He has also acted in soap operas Brookside, Casualty and the short-lived revival of Crossroads. Dennis also appeared in the second series of the UK Celebrity Big Brother.
In 2005 Les appeared as a guest star in the BBC-series Extras starring Ricky Gervais, playing himself and making several jokes about his career, or the lack thereof. He received praise from reviewers for being so honest and open (and being willing to be the punchline of jokes about his apparent lack of success). He was even rumoured to have rejected the original script, claiming that it was too tame. . In September and October 2005 he appeared in Ant & Dec's Gameshow Marathon, in which he visited the winners at home to present the prizes they had won.
In 2006, Dennis presented and performed in the BBC's 'The Sound of Musicals'.
More recently he has appeared on TV as both as host and actor, as well as touring in numerous theatre productions ranging from Shakespeare to musicals including 'Hairspray' and 'Legally Blonde'.