The cause of death was not immediately known.
Van Lier, who most recently worked for Comcast SportsNet Chicago, had been scheduled as a pre- and post-game analyst for Wednesday night's Bulls game with the New Jersey Nets.
Comcast officials became concerned after Van Lier did not show up for work and could not be reached Thursday, so an employee was sent to his apartment near the United Center on the city's near West Side, said Jim Corno, president of Comcast SportsNet Chicago. The employee tried unsuccessfully to get inside.
Authorities responding to a subsequent well-being check request found Van Lier unresponsive shortly before 1 p.m. He was pronounced dead at the scene, said Chicago Fire Department spokesman Quention Curtis.
The Cook County Medical Examiner's office confirmed Van Lier's death but said a cause was not immediately known.
"Norm Van Lier was one of the all-time greats ever to put on a Chicago Bulls uniform," Bulls chairman Jerry Reinsdorf in a statement. "Along with Jerry Sloan, he set a standard for Bulls defense and toughness which we will never forget and which we will always strive to replicate."
Van Lier began his NBA career with Cincinnati in 1969. He later spent more than six seasons with the Chicago Bulls before finishing his career with Milwaukee in 1979.
A three-time All-Star, Van Lier played on five playoff teams.
It was Van Lier's time with the Bulls for which he was most vividly remembered.
"He's leaving an incredible legacy behind -- great accomplishments as well as a charitable person," said Artis Gilmore, his teammate in Chicago.
Van Lier was a defensive standout and a fan favorite who was given the name "Stormin' Norman" for his fiery play and defensive tenacity.
Named to the NBA's All-Defensive First or Second teams eight times, Van Lier and longtime teammate Jerry Sloan, now the coach of the Utah Jazz, formed one of the top defensive guard tandems in NBA history.
"Norm was as tough as anybody," Gilmore said. "If you look at the players today, they're big and strong and they're very talented. Norm, it didn't matter. He was a small individual and would take the challenge. He would take the charge from Wilt Chamberlain."
On Thursday, Gilmore was in "total disbelief and shock."
"The last picture I have with Norm he was laughing and having a good time," Gilmore said, recalling a Bulls charity golf outing in the spring.
Van Lier retired after the 1979 season with 8,770 points and 5,217 assists. He finished among the league's top 10 in assists eight times, and among the league's top 10 in steals per game three times, according to Basketball-Reference.com.
"Norm was a complete player, a wonderful passer, a tenacious rebounder and an original character in the 1970s," NBA Commissioner David Stern said in a statement. "We are all fortunate that Norm continued to share his passion and insight as a broadcaster for the Bulls since the early '90s."
Jimmy Collins, the Bulls' first-round draft pick in 1970, described Van Lier as a "hard-nosed fun-loving guy" with a tendency to sing Rolling Stones songs -- particularly "Satisfaction" -- when he wasn't shutting down opponents.
"He loved to sing," said Collins, the head coach at Illinois-Chicago. "I was in love with the Temptations. He could tell you every hard rock artist out there. I could tell you every Temptations song."
Van Lier the analyst remained a huge Bulls fan, albeit at times a critical one who would call out players he did not think were playing hard or smart.
"If he felt like a guy was not giving maximum effort and not playing the right way he'd let him have it in the post game show," said Mark Schanowski, the host of the pre- and post-game shows who worked with Van Lier for four years.
"He wore the Bulls on his sleeve," Corno said. "When the Bulls played well, Norm was good and happy enough. When they didn't play well, he felt it."
Associated Press Sports Writer Andrew Seligman contributed to this report.