He attended the prestigious Boston Latin School and graduated at the top of his class, which won him a position at Harvard College. He completed his B.A. in three years when the Board of Overseers of Harvard conferred him his degree. Afterward, Redstone served in World War II, decoding Japanese messages for the United States Army. Upon completion of his Army service, he worked in Washington, D.C. and attended Georgetown University Law School. He chose to transfer into Harvard Law School and received his LL.B from that institution.
After completing law school, Redstone worked primarily in Washington, D.C., working at first for the U.S. Department of Justice in San Francisco and then going into private practice. However, after a few years in practice, he chose to join his father's theater chain management operation, what is now known as National Amusements.
As Redstone grew National Amusements, he believed that content would become more important than distribution mechanisms. There would always exist channels of distribution (albeit in varied forms), but content was always going to be necessary (his famous quote is "content is king!"). He then made investments in Columbia Pictures, Twentieth-Century Fox, Orion Pictures, and Paramount Pictures, all of which turned over huge profits when he chose to sell the stock in the early 1980s.
Looking for a new business to develop, he set his sights on Viacom International, a company that was a spin-off of CBS in 1971 after the FCC ruled that television networks could not syndicate programs they produced. Viacom syndicated most of CBS's programs, but also made a lot of money from syndicating other programs, including most of Carsey-Werner Productions' shows (The Cosby Show, Roseanne, and A Different World), as well as syndicating shows for other companies (Columbia Pictures Television's All In The Family was one notable example). Viacom also owned MTV Networks (formerly known as Warner-AMEX Satellite Entertainment), which owned MTV and Nickelodeon. In addition, other included properties included Showtime Networks (a similar pay-television network to HBO and Cinemax) and The Movie Channel. Viacom acquired MTV Networks in 1985 for $550 million from Steve Ross' Warner Communications (WCI bought American Express' share and then sold the entire entity to Viacom, as they felt that they could not make a lot of money from the venture and the bias that a studio owning cable channels would be a conflict of interest, which would change when Time Warner bought Turner Broadcasting in 1995).
After a hostile takeover in 1987, Redstone won voting control of Viacom and led a series of acquisitions to make Viacom one of the top players in modern media (along with General Electric's NBC-Universal, News Corporation (parent of Fox), Time Warner, Sony, and The Walt Disney Company).
Redstone's next acquisition came in the form of the purchase of Paramount Communications, parent of Paramount Pictures, in 1993, which he fought over with Barry Diller (former board member of Vivendi Universal and CEO of IAC/InterActiveCorp) and John Malone (president of TCI/Liberty Media), where he had to raise his bid three times. Some say that Redstone overpaid, but after he shed certain assets (the Madison Square Garden properties to Chuck Dolan's Cablevision and Simon & Schuster's educational publishing units to Pearson plc for almost $4 billion), Redstone turned Paramount's expenditure into a substantial profit.
The Paramount acquisition was only the tip of the iceberg. Redstone purchased Blockbuster Entertainment, which included Aaron Spelling's production company and a huge library of films, much of which has been merged into Paramount Pictures. Blockbuster has now been spun off into its own independent entity.
The final acquisition came in the form of Viacom's former parent, CBS. Former Viacom President & COO Mel Karmazin (who was then the President of CBS) proposed a merger to Redstone on favorable terms and after the merger completed in 2000, Viacom had some of the most diversified businesses imaginable. Viacom had assets in the form of broadcast networks (CBS and UPN), cable television networks (MTV, Nickelodeon, MTV2, Comedy Central, BET, Nick at Nite, Noggin/The-N, TV Land, CMT, and Spike TV), pay television (Showtime and The Movie Channel), radio (Infinity Broadcasting, which produces Howard Stern and Don Imus' radio shows), outdoor advertising, music publishing (Famous Music), motion pictures (Paramount Pictures), and television production (Spelling Entertainment, Paramount Television, and Big Ticket Entertainment), and King World Productions (a syndication unit, which notably syndicates runaway daytime hit, The Oprah Winfrey Show, as well as Dr. Phil, Wheel of Fortune, and Jeoprardy), among others.
Redstone made arrangements to step down from Viacom in 2006, mainly due to concerns about his age. After Mel Karmazin resigned in 2004, two heir apparents were named: Co-President & Co-COO Leslie Moonves (who was #2 to Karmazin at CBS; he was the former head of Warner Bros. Television and before that, Lorimar Television) and Co-President & Co-COO Tom Freston (who has been President & CEO of MTV Networks since 1987 and had been with the company since the formation of MTV Networks' precursor company, Warner-AMEX Satellite Entertainment).
It is rumored that Moonves was promoted to Co-President & Co-COO with Tom Freston, not only for his talent and management style, but also because he was on the short list of executives to replace Michael Eisner at the Walt Disney Company when his contract runs out in 2006 (others speculate that Peter Chernin, the President & COO of News Corporation, parent of Fox Entertainment Group, is at the top of that list). To keep Moonves on board, Redstone elevated his position to equal standing with Tom Freston.
Moonves oversees CBS, UPN, Infinity Broadcasting, Viacom Outdoor, and Paramount Television, while Freston oversees MTV Networks, Showtime Networks, Paramount Pictures, and Simon & Schuster.
Tom Freston's long service with Viacom was believed to make him the most likely successor to Redstone's job as Viacom CEO, although it is still uncertain.
Complicating matters further, on March 16, 2005, Sumner announced that he and the Viacom board are looking into a potential split of the company. Under terms that Sumner floated, Freston would run a company that primarily consisted of MTV Networks, and Moonves would run a company that primarily consisted of CBS, Viacom's billboard operations, and its radio holdings. 
The company split was approved by the Viacom board on June 14, 2005.
Sumner also owns over eighty percent of Midway Games, both individually and through National Amusements.
His biography, A Passion To Win (which was co-written by Peter Knobler), was released in 2001 and published by Viacom's Simon & Schuster book publishing company. This book details everything from Sumner's life as a young boy in Boston, to the difficult takeover of Viacom, and the problems he overcame in purchasing and managing both Blockbuster Video and Paramount Pictures. There is also coverage of the legendary CBS merger (Viacom was a spin-off company of CBS to syndicate its programs, and the subsidiary bought the parent almost 30 years later!).