Haim broke into acting at the age of 10, playing the role of Larry in the Canadian children's educational comedy television series The Edison Twins, which ran from 1982 until 1986. He made his feature film debut in 1984's thriller Firstborn, as a boy whose family comes under threat from his mother's violent boyfriend, played by Peter Weller. Haim's first day of shooting was with Weller, and he went to compliment the older actor on his performance. Weller collared Haim, throwing him up against a wall to warn him not to speak to him after a take. It took three assistants to separate them.