He is best known for his roles as Donald "Boon" Schoenstein in Animal House (1978), oil company executive "Mac" MacIntyre in Local Hero (1983), pickle store owner Sam Posner in Crossing Delancey (1988) and Lt. Mitch Kellaway in The Mask (1994).

He worked at a number of jobs, including teaching, waiting tables, and as a social worker before settling on acting as a career.

Off-Broadway he has appeared in Road to Nirvana, The Birthday Party, Isn't It Romantic, Sexual Perversity in Chicago, and A Rosen by Any Other Name.

He starred opposite former girlfriend Bette Midler in the television adaptation of Gypsy and was featured in the HBO drama Barbarians at the Gate (which earned him an Emmy Award nomination for Outstanding Supporting Actor in a Miniseries or a Special).

In 2011, Riegert began a multi-episode arc on One Tree Hill as August Kellerman, Nathan's unforgiving college professor.

Riegert narrated the audiobook of Michael Chabon's The Yiddish Policemen's Union, which was nominated for a 2008 Audie Award in literary fiction.