James gerard Dinan was born to a Roman Catholic family in 1959 in Baltimore, Maryland, one of five children of Robert and Jeannette Dinan. His Father was a textile Engineer and his mother a homemaker. In 1969, the family moved to Worcester, Massachusetts. Dinan graduated from the private Bancroft School and then went on to earn a B.S. from the Wharton School of the University of Pennsylvania with a degree in economics. While at the University of Pennsylvania, Dinan joined Alpha Chi Rho, a northeastern fraternity. In 1981, he took a job with stock research firm, Donaldson, Lufkin & Jenrette (DLJ) and later earned a M.B.A. from Harvard University.

In 1985, he took a job at the merger arbitrage firm Kellner DiLeo & Company. In 1987, the market crashed and he lost his entire $600,000 in savings. In 1991, he was able to raise $3.6 million from his former DLJ colleagues and started his own hedge fund named York Capital (named after the street he was then living on, York Avenue). In 1993, his fund earned credibility with a 33.8 percent return and by 2000, the fund had over $610 million in assets. In 2010, he sold 33% of York to Credit Suisse for $425 million. In 2011, he made a $1 million donation to the Museum of the City of New York. In July 2014, Dinan gained partial ownership of the NBA's Milwaukee Bucks.