Haas was discovered at the age of five in his kindergarten by casting director Margery Simkin. His first screen role was as a child in the 1983 nuclear holocaust film "Testament". He became more widely known in 1985 when, at the age of eight, he appeared in "Witness". His performance as an Amish boy who is the sole witness to a police officer's murder was well received by critics. Haas followed this with roles in films such as "Lady in White" and "Solarbabies". In 1989, he appeared in the film "Music Box", as the 12-year-old son of Jessica Lange's character, and grandson of an immigrant who is accused of being a war criminal. For this role he was nominated for the Young Artist Award. He also starred in the films "Alan and Naomi" and "Rambling Rose".