In 1967, using earnings from his investments, Steinhardt founded the hedge fund Steinhardt, Fine, and Berkowitz (later Steinhardt Partners) with co-investors William Salomon, former managing partner of Salomon Brothers and Jack Nash, founder of Odyssey Partners. Steinhardt Partners averaged an annualized return for its clients of 24.5%, after a 1% management fee and a "performance fee" of 15% (early in his career, later 20%) of all annual gains, realized and unrealized, nearly triple the annualized performance of the S&P 500 Index over the same timeframe. After decades of successfully managing the fund, Steinhardt and his firm were investigated for allegedly trying to manipulate the short-term Treasury Note market in the early 1990s. He personally paid 75% of a total fine of $70 million as part of settlement with the U.S. Securities and Exchange Commission and Department of Justice. His firm made $600 million on the Treasury positions. In his book, "No Bull," Steinhardt said he did nothing wrong, but merely settled the case in order to "move on." Following a negative performance in his hedge fund in 1994, Steinhardt Partners enjoyed an excellent 1995, with performance in line with its historical record. The hedge fund closed and distributed all monies to its limited partners at the end of 1995.
In 1967, Steinhardt met his future wife Judy in a car pool he organized. During one of the carpools into New York City, he mentioned the name of the grain elevator company Colorado Milling and Grain Elevator. Judy mentioned the company to her father who invested in the company and made a substantial profit after Colorado Milling was acquired by Great Western Sugar in 1968; the merged company was renamed Great Western United. Judy is the chairwoman of New York University's Institute of Fine Arts and of the American Friends of the Israel Museum. They have three children: David, Daniel and Sara. His daughter, Sara, a former journalist and parenting columnist for The New York Sun, is married to Mark Bloom and is the president of the Hebrew Language Academy Charter School in Brooklyn.