Harrington is a former champion backgammon player, U.S. chess master (he won the 1971 Massachusetts State Chess Championship), and bankruptcy lawyer. During his time at Suffolk University, he was part of an MIT team that gained an advantage over casinos at roulette. Shortly after the MIT team disbanded he was part of a different one which specialized in blackjack. 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-80's where he played with Howard Lederer and Erik Seidel.
He won the World Series of Poker (WSOP) main event in 1995 for $1,000,000 and sat at three other final tables at the main event, placing 6th in 1987 for $43,750, 3rd in 2003 for $650,000, and 4th in 2004 for $1,500,000. 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.
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. According to Barry Greenstein.