In 1993, he portrayed George Costanza on the show-within-a-show scene in the Seinfeld episode, "The Pilot". Piven was a supporting cast member on the last three seasons of the sitcom Ellen, where he played the part of Ellen's cousin, Spence. He was also the star and producer of 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 playing fast-talking, acerbic Hollywood agent Ari Gold in the HBO series Entourage. He received Emmy nominations for Best Supporting Actor four straight years, from 2005 to 2008, winning the award in 2006, 2007, and 2008; 2009 was the first year in which he did not receive an Emmy Award nomination for his work on the show. He also received Golden Globe nominations from 2005 to 2010 for the role, winning in 2008. Piven has appeared in numerous successful films, including Lucas, 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. He has also made cameo appearances in Rush Hour 2 and the US release of Cars.

In 2007, Piven appeared in the video for "Drivin' Me Wild", the third single taken from rapper Common's seventh album Finding Forever. The two were co-stars in Smokin' Aces, and appeared together when Piven hosted SNL in January 2007.

Piven appeared in the first Broadway revival of David Mamet's Speed-the-Plow, co-starring Mad Men star Elisabeth Moss and three-time Tony nominee Raul Esparza. The production began preview performances on October 3, 2008, and opened on October 23, 2008; the play was due to run through February 22, 2009. After Piven missed several performances, on December 17, 2008, Piven's rep. announced that due to an undisclosed illness, Piven would be ending his run in the play effective immediately. The illness was revealed to be hydrargaria, a disease caused by exposure to mercury or its compounds, though the source is unknown. Rumours have indicated that the high level of mercury could potentially have been caused by Piven's habit of consuming fish twice a day for the past 20 years. An alternative explanation is that the herbal remedies Piven was taking were responsible for his high levels of mercury. Mamet joked that Piven was leaving the play "to pursue a career as a thermometer." 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. William H. Macy and Norbert Leo Butz replaced Piven in the Broadway show.

In 2017 he will star in the CBS drama Wisdom of the Crowd.

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