Sobel was born in the Bronx, in New York City, New York. He completed his B.S.S. (1951) and M.A. (1952) at City College of New York, and after serving in the U.S. Army, obtained a PhD from New York University in 1957. He started teaching at Hofstra in 1956. Sobel eventually became Lawrence Stessin Distinguished Professor of Business History at Hofstra. After his death, the university established the Robert Sobel Endowed Scholarship for Excellence in Business History and Finance.
Sobel's first business history, published in 1965, was The Big Board: A History of the New York Stock Market. It was the first history of the stock market written in over a generation. The book was met with favorable reviews, and solid sales, and Sobel's writing career was launched. Several of his subsequent books were best sellers.
Besides writing more than 30 books, Sobel authored many articles, book reviews, and scripts for television documentaries and mini-series. From 1972 to 1988, Sobel's weekly investment column, "Knowing the Street," was nationally syndicated through New York Newsday. He was also regularly published in national periodicals, including The New York Times and The Wall Street Journal. At the time of his death, Sobel was also a contributing editor to Barron's Magazine. He was a regular guest on financial and other news shows, such as Wall Street Week and Crossfire.
Sobel was perhaps most famous for his only work of fiction, the 1973 book, For Want of a Nail. This book is an alternate history in which Burgoyne won the Battle of Saratoga during the American Revolutionary War. This work detailed the history of an alternate timeline, complete with footnotes. Sobel had authored, or co-authored, several actual text books. For Want of a Nail was republished in 1988 and won several science fiction awards.
Sobel's dominant passion was Wall Street, a metaphysical neighborhood that had fascinated him since childhood. "It is as though you are walking through a historical theme park, with this engaging man at your side pointing out the sights," said Andrew Tobias, the author and investment guide, in a review in The New York Times of The Last Bull Market: Wall Street in the 1960s (W. W. Norton, 1978).
Most of Sobel's books were written for a general audience, but he never bristled when some scholarly writers dismissed him as a "popularizer," said his colleague and friend George David Smith, a professor of economic history at New York University. "Quite the contrary -- he saw that as his mission in life."