Le Vaillant began acting soon after the family returned to England, although he initially struggled to find work in the industry and spent nine years unemployed. After short appearances, such as P.C. Miller in The Gentle Touch in 1980, he spent three series in the BBC One drama Casualty playing the character of Dr. Julian Chapman, before leaving to star as the title character in the prime-time BBC series Dangerfield. He left the production in 1997, having married former Casualty star Nicola Jeffries two years earlier. He also appeared in the short-lived sitcom Honey for Tea, and in the cinema film Tom’s Midnight Garden, playing the adult Tom.

He briefly returned to acting in 2009 to play British Prime Minister Edward Heath in the television drama Margaret.