Riegert was born in the Bronx, the son of Lucille, a piano teacher, and Milton Riegert, a food wholesaler. Riegert grew up in Hartsdale, New York, and was raised in a non-observant Jewish household. He graduated from Ardsley High School in 1964 and later from the University at Buffalo. He worked at a number of jobs, including teaching, waiting tables, and as a social worker before settling on acting as a career.

Riegert made his television debut in two episodes of M*A*S*H. He has also portrayed Newark Assemblyman and later New Jersey State Senator Ronald Zellman in The Sopranos and defense attorney Chauncey Zeirko in multiple episodes of Law & Order: Special Victims Unit. 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), then he starred as composer Jake Rubin in the miniseries Ellis Island, the final episode of Seinfeld as the president of NBC and the television movie Back When We Were Grownups, and he voiced the character of Max Weinstein in the controversial episode “When You Wish Upon a Weinstein” of Family Guy. Riegert guest-starred in a Season 2 episode of Leverage as corrupt lawyer Peter Blanchard. In 2011, Riegert began a multi-episode arc on One Tree Hill as August Kellerman, Nathan’s unforgiving college professor. Riegert appeared as the character George Moore in Season 1 of Damages. He also appears in a recurring role as Judge Harvey Winter in CBS’ The Good Wife. He appeared in Dads as David Sachs, the father of Eli Sachs, played by Seth Green. Riegert also appeared in Seasons 3 and 4 of Unbreakable Kimmy Schmidt. He made a multi-episode guest appearance in the second half of the Netflix original comedy series Disjointed.