Baron worked for several brokerage firms from 1970 to 1982. During this time, he developed a reputation for investing in small, unloved companies.

In 1978, Baron married Judy Bernard in a Jewish ceremony at the Harmonie Club. In 2007, he paid $103 million for a house in East Hampton, New York—the most ever paid for a residential property at that time—from Adelaide de Menil, heiress to the Schlumberger fortune. de Menil's house had been built by piecing together historic East Hampton buildings that she moved to the property to protect them from demolition. Prior to the close of the sale, de Menil broke up the structures and moved them to various locations in the town for protection, including six that were moved a mile north to where they will form the new campus of the East Hampton Town government. Baron is now completing a new 28,000-square-foot (2,600 m) house, designed by Hart Howerton, a New York architectural firm with several other projects in the Hamptons, which specializes in large-scale land use. The house was included in a 2008 Vanity Fair article.

He founded Baron Capital Management in 1982. Baron Capital is well known for its long-term strategy and investment outlook. The firm will typically hold a stock for 4–5 years, sometimes as many as 10–15 years. The firm prefers to invest in mega-trends driven by broad societal and demographic trends, including baby boomer demands for Health care - trends where demand is expected to remain steady for years, or even decades. The firm seeks to invest in companies that have strong management teams, investing in people not assets. They look for companies that have strong growth opportunities, are appropriately financed, have competitive advantages, and are a leader in their field. Baron Capital places a unique focus on the strength of the management teams of the companies they invest in, looking for credible, dependable, trustworthy leadership. In 2011, Baron Capital had approximately $19.5 billion in assets under management. In an effort to thank Investors, Baron hosts an annual shareholder meeting which typically features rock acts such as Elton John, the Beach Boys, and Lionel Richie. In 2012, Baron Capital purchased 24 percent of the stock of the Manchester United Football Club that was offered on the New York Stock Exchange by the Glazer family. Baron's investment amounts to a 2.5% ownership interest in the club as only 10% of the team's stock was floated.