[10] As a teenager, he attended Harand Theater Camp in Elkhart Lake, Wisconsin, where he played Bernardo in West Side Story.

His first important role came in 1992 when he became a regular cast member on HBO's The Larry Sanders Show, where he played head writer Jerry.

He also starred in and produced the short-lived ABC dramedy series Cupid, and voiced Elongated Man in three episodes of Justice League Unlimited.

Beginning in 2004, Piven achieved significant success as the fast-talking, acerbic Hollywood agent Ari Gold in the HBO series Entourage.

Piven has appeared in numerous films, including Judgment Night, Grosse Pointe Blank, Singles, Very Bad Things, The Family Man, Black Hawk Down, The Kingdom, Heat, PCU, Old School, Rock N Rolla, Serendipity, Smokin' Aces, and Runaway Jury.

"[19] On September 1, 2009, Piven, in a guest appearance on the Late Show with David Letterman, explained that he had given up red meat and poultry, and had been getting all of his protein from fish for the past 20 years.

On August 1, 2019, Piven appeared on a KTVU morning news segment to promote a standup comedy show he was to perform in the next night in San Francisco.

[24] On October 30, 2017, adult film actress and reality television personality Ariane Bellamar[25] made accusations via Twitter that Piven had groped her.

[27] Cassidy Freeman came to Bellamar's defense in an Instagram post, stating that Piven engaged in "predatory behavior" toward her when she was "far too young".

[28] On November 9, advertising executive Tiffany Bacon Scourby alleged that Piven had attacked her in 2003, stating that he had "jumped on" her, "exposed his genitals, held her hands down and began rubbing against her body until he ejaculated".

"[31][32] Later that month, Anastasia Taneie, who worked as an extra on Entourage, alleged that Piven "confronted her in a dark hallway and groped her breast and genitals as he forcefully pushed her against a wall".