It was founded as a private equity firm in 1998 by Wes Edens, Rob Kauffman, Pete Briger, Michael Novogratz, and Randal Nardone.

In December 2017, Fortress was fully acquired by SoftBank Group, was delisted, and returned to being a privately held company.

[2] In May 2023, Mubadala Investment Company agreed to acquire a majority stake in Fortress from Softbank for an undisclosed sum.

[5] [6] The firm quickly expanded into hedge funds, real estate-related investments and debt securities, with Michael Novogratz leading the hedge fund strategy and Peter Briger, also a former Goldman Sachs partner, overseeing the credit and real estate-related investments.

[8] When Fortress launched on the NYSE on February 9, 2007, with Goldman Sachs and Lehman Brothers underwriting the IPO, it was the first large private equity firm in the United States to be traded publicly.

[11] Fortress and its principals were subsequently featured in an April 2009 Vanity Fair article on the adverse economic conditions facing hedge funds.

[19][20] The SoftBank acquisition was completed in the last week of December,[21] with Fortress being delisted from the New York Stock Exchange, and returned to being a privately held company.

The route is being advertised as “A Brightline Company.” On January 3, 2018, it was reported that Fortress was nearing a deal to sell its stake on OneMain to Apollo Global Management.

[33] On May 10, 2024, the Financial Times reported the Abu Dhabi sovereign investor Mubadala Capital's $3 billion bid for New York-based Fortress Investment Group has cleared significant U.S. regulatory hurdle after the parties agreed to important concessions.

The Committee on Foreign Investment in the United States (CFIUS) approved Fortress' sale of a majority equity interest to Mubadala, the newspaper said, citing three people familiar with the matter.

[citation needed][35] In November 2006, RailAmerica announced that a Fortress-managed fund would acquire the company, offering $16.35 per share (a 32% premium).

Fortress has said it lost $125 million purchasing fraudulent promissory notes from Marc Dreier, who had been operating a Ponzi scheme.

[41] The lawsuit was filed in New York state court, alleging Dechert issued a "false" legal opinion letter that Dreier used to defraud Fortress.

[42] Fortress's private equity investment portfolio includes Aircastle Limited, Alea Group Holdings (Bermuda) Ltd., AMRESCO, Boxclever, Capstead Mortgage Corporation, CW Financial Services, Eurocastle Investment Limited, Flagler, Florida East Coast Railway, GAGFAH, GateHouse Media, Inc., Global Signal, Inc., Green Tree Servicing LLC, Holiday Retirement, Intrawest, Italfondiario, Kramer Junction, Mapeley Limited, MBS Holdings, MS Hub, Nationstar Mortgage LLC, Penn National Gaming, Inc., Prime Retail, RailAmerica, RESG, Seacastle Inc., Simon Storage, Springleaf Financial, Umami Burger, and IPCom GmbH & Co.

[44] In March 2014, John E. Giles of Great Lakes Partners estimated a $10–$20 million investment would be needed over three years to repair the line, which Fortress brands as the Central Maine and Quebec Railway, as it is in poor condition and currently not safe for the transport of oil or dangerous goods.

[46] In October 2014, it was reported by the Birmingham Business Journal that Fortress had purchased the Inverness Corners retail center.

[49] In August a New York Supreme Court judge approved proceeding with a foreclosure auction against properties owned by Cohen to recoup part of the value of the loans.

[54] Three weeks before the opening of the 2010 Winter Olympics in Vancouver, Fortress failed to make payment on its loan used to buy out Intrawest.