Harrington was born in Cambridge, Massachusetts just outside of Boston. Currently residing in Santa Monica, California, Harrington is a former champion backgammon player and U.S. chess master (he won the 1971 Massachusetts State Chess Championship). Before becomming a poker pro and businessman, Harrington worked for many years as a bankruptcy lawyer. He also played poker against Bill Gates while Gates was at Harvard. Some of his earlier poker experience came from the Mayfair Club in the mid-1980s where he played with Howard Lederer, Steve Zolotow, and Erik Seidel.

In addition to being a successful professional poker player, Harrington also works in real estate and the stock market. Currently, Harrington also owns and operates his own company, Anchor Loans, which makes various types of loans and has suffered no loss since its creation, according to an interview given by Harrington. He has also stated during interviews about his poker career, that he is there for the money, not the fame or glory, unlike some poker players. Also unlike many professional poker players, Dan considers himself only a part-time player, since he spends a large amount of his time on his business interests. He usually only plays a few events at the World Series of Poker each year and plays in occasional World Poker Tour events and a few other tournaments.

Harrington first cashed in the World Series of Poker in 1986. The next year, in only his second WSOP cash, Harrington made the final table of the Main Event. He finished in 6th place in the event which was won by Johnny Chan. Harrington's friend and fellow Mayfair Club member, Howard Lederer was also at this final table, finishing in 5th place.

Sporting his iconic green Boston Red Sox cap, Dan Harrington is known as a crafty, tight-aggressive player, employing starting hand standards that are stricter than most professionals. When he reached the final table at the 1995 main event, he set the runner-up, Howard Goldfarb, to bluff for all his chips in the final hand.

His solid play allows him to make it to many final tables at large events. He won the World Series of Poker (WSOP) main event in 1995 for $1,000,000. He has made three other main event final tables: placing 6th in 1987 for $43,750, 3rd (out of 839 players) in 2003 for $650,000, and 4th (out of 2,576 players) in 2004 for $1,500,000. Because of the increased number of participants, his run of back-to-back main event final tables in 2003-04 has been called the greatest accomplishment in World Series history, particularly by ESPN poker announcer Norman Chad. As defending champion in 1996, Harrington made another deep run in the main event, finishing in 17th place and earning $23,400. He also cashed in the 2009 main event, finishing in 252nd place for a $32,963 payout.

The same year as his main event win, he also won a bracelet in the $2,500 No-Limit Hold'em event for $249,000 and the Seven-card stud event at European Poker Open in London. He made his first final table at the World Poker Tour (WPT) in 2005, winning $620,730 for his second place finish to Minh Ly in the Doyle Brunson North American Championship. In 2007, he won the Legends of Poker for a prize of $1,634,865.

In 2008, Harrington made his first appearance on the NBC show Poker After Dark in the episode titled "Mayfair Club." This tournament brought together poker players who had played at the legendary Mayfair Club in New York City. The other players in the tournament with Harrington included professional poker players Howard Lederer, Mickey Appleman, Steve Zolotow, Jay Heimowitz, and Mike Shictman, the owner of the Mayfair Club for many years. Harrington busted out first, finishing in 6th place. The tournament was won by Jay Heimowitz who defeated Howard Lederer in heads-up play.

Harrington, Doyle Brunson, Carlos Mortensen, Scotty Nguyen and Joe Hachem are the only five people to have won the World Series of Poker Main Event and a World Poker Tour title.

As of 2010, his live tournament winnings exceed $6,600,000. More than half of his live tournament winnings ($3,524,476) have come at the WSOP.

He has written (co-authored with Bill Robertie) three popular books on tournament poker and two books on cash no-limit games, all published by Two Plus Two Publishing:

• Harrington on Hold'em: Volume I: Strategic Play ISBN 1-880685-33-7 (2004)
• Harrington on Hold'em: Volume II: The Endgame ISBN 1-880685-35-3 (2005)
• Harrington on Hold'em: Volume III: The Workbook ISBN 1-880685-36-1 (2006)
• Harrington on Cash Games, Volume I: How to Play No-Limit Hold 'em Cash Games ISBN 1-880685-42-6 (2008)
• Harrington on Cash Games, Volume II: How to Play No-Limit Hold 'em Cash Games ISBN 1-880685-43-4 (2008)