Willard died Monday. He had officiated nearly 1,500 regular-season games when he left the game in 2012 after being was diagnosed with pancreatic cancer. The NBA said the cancer was the cause of Willard's death.
"The entire NBA family joins me in mourning the loss of one of our own, Greg Willard," NBA Commissioner David Stern said in a statement.
"Greg touched all those with whom he came in contact thanks to his extraordinary spirit, dedication and hard work. As a Finals referee, he reached the highest level of his profession while at the same time demonstrating a strong commitment to his family and his community.
"Our thoughts are with his wife Laurie, their children and the rest of the Willard family during this difficult time."
NBA referees will honor Willard's memory by wearing wristbands or patches with his jersey number 57 for the remainder of this season.
The NBA said Willard called 1,494 regular-season games and 136 playoff games. He was on the officiating crew for two NBA Finals and the 2006 NBA All-Star Game.
"Greg epitomized what it meant to be an NBA referee through dedication to his craft, hard work, and integrity both on-and-off the court," said National Basketball Referees Association General Counsel Lee Seham. "He was not only a great NBA referee but, more importantly, a wonderful person and our thoughts and prayers are with his family during this difficult time."