After graduating from college, Feinberg worked as a trader at Drexel Burnham in 1982 and later at Gruntal & Co..
In 1992, at the age of 32, Feinberg co-founded Cerberus Capital Management with william L. Richter. At the time the firm had $10 million under management; its assets under management have since grown to over $30 billion in 2016. In 1999, the firm hired former Vice President Dan Quayle as a Chairman of Cerberus Global Investment. In 2006, the firm hired former United States Secretary of the Treasury John Snow, who serves as a Chairman of Cerberus.
Feinberg reportedly made $50 million in 2004. His lifestyle is less extravagant than his peers in private equity. He splits time between his homes on Manhattan's Upper East Side and Greenwich, Connecticut with his wife Gisela (née Sanchez).
Feinberg has been described as "secretive" in The New York Times. In 2007, Feinberg told Cerberus shareholders, "If anyone at Cerberus has his picture in the paper and a picture of his apartment, we will do more than fire that person. We will kill him. The jail sentence will be worth it."
Feinberg has been critical about the pay received by private equity executives, stating, "In general, I think that all of us are way overpaid in this Business. It is almost embarrassing." He has also noted in comments made in 2011 that smaller private equity fund sizes may be better for investor returns: "If your goal is to maximize your return as opposed to assets under management, I think you can be most effective with a big company infrastructure and a little bit smaller fund size."
Feinberg is a major Republican donor. In 2016, he served on the Trump Economic Advisory Council during Donald Trump's presidential campaign, donated nearly $1.5 million to pro-Trump PACs, and co-hosted a $50,000 per person Republican National Committee and Trump fundraising dinner alongside other financiers. In February 2017, the New York Times reported that President Trump will assign Feinberg a role in the White House leading a review of the US intelligence agencies.