Sadler was born in Toronto, Canada, the son of a German graphic designer and a British teacher.

After studying at the Royal Academy of Dramatic Art in London, his career as actor started in 1994 with roles in German thrillers such as Wolffs Revier and SK-Babies.

In 2002 he worked on two further religious films: The Bible – The Apocalypse with Richard Harris and Luther with Joseph Fiennes and Sir Peter Ustinov.

In 2007 he starred in Contergan [de] as a lawyer and father of a thalidomide-disabled daughter, for which he received a Bambi Award in 2007.

He explained in an interview that he was proud to be a part of the film, which influenced the German parliament to pass a law giving greater financial compensation for victims of thalidomide.