He won gold medals with Team USA at the 1975 Pan American Games and the 1976 Summer Olympics.

Born in Satu Mare, Romania, Grunfeld immigrated with his parents, Alex and Livia, to the United States in 1964 when he was eight years old.

[5][6] He attended the University of Tennessee, where he played basketball with future NBA Hall of Famer Bernard King.

On December 26, 1978, Grunfeld led all scorers with 27 points in a 143–84 blowout victory against the Detroit Pistons.

Grunfeld was selected to participate as a member of the American basketball team at the 1973 Maccabiah Games, while he was still attending high school.

He then briefly worked under Stu Jackson as an assistant coach for the Knicks before starting his career in team administration.

[8] Grunfeld was appointed director of administration in the 1990–91 season and was moved to vice-president of player personnel on April 23, 1991.

[8] At the time of his removal from the general manager post during the 1998–99 season, the Knicks had a 21–21 record and were on the verge of not making the playoffs.

Before the start of the season, he organized the trade of Charles Oakley to the Toronto Raptors for Marcus Camby, and John Starks to the Golden State Warriors for Latrell Sprewell.

But at the season's end, the Knicks having reached the NBA Finals, MSG announced that all was forgiven and Grunfeld would be reinstated.

In 2004, Grunfeld traded the number five pick in the 2004 NBA draft along with Jerry Stackhouse for All-Star Antawn Jamison.

In the 2007 and 2008 NBA draft classes, Grunfeld selected Nick Young and JaVale McGee respectively.

In the 2009 NBA draft, Grunfeld traded the team's first-round pick (5th overall) for Mike Miller and Randy Foye, both of whom only spent one season in Washington.

In July 2014, he signed former NBA Finals MVP Paul Pierce to replace Trevor Ariza, Kris Humphries in a sign and trade with the Boston Celtics (trading a 2015 2nd round pick), and DeJuan Blair in free agency.

Later on that same day, he traded Austin Rivers and Kelly Oubre Jr. to the Phoenix Suns for the return of Trevor Ariza.