"Dennis was just an awesome player," his former Celtics coach K.C. Jones said. "He played hard and he took the big shots."
Johnson, coach of the Austin Toros, was talking with a team staff member when he collapsed outside the Austin Convention Center. He was unconscious and in cardiac arrest when paramedics arrived, said Warren Hassinger, spokesman for Austin-Travis County Emergency Medical Services.
Paramedics tried to resuscitate him for 23 minutes before he was taken to a hospital and pronounced dead, Hassinger added. Mayra Freeman, a spokeswoman for the medical examiner's office, said there will be an autopsy.
The Toros postponed home games Friday and Saturday nights, the NBA Development League said.
A five-time All-Star and one of the NBA's top defensive guards, Johnson was part of the last Boston dynasty. He spent 14 seasons in the league and retired after the 1989-90 season. He played on title teams with the Celtics in 1984 and 1986 and with the Seattle SuperSonics in 1979, when he was the NBA finals MVP.
"Dennis was a great player, one of the best teammates I ever had, and a wonderful person," said former Celtics teammate Larry Bird, now president of the Indiana Pacers. "My thoughts and condolences are with his family at this difficult time."
Toros owner David Khan said Johnson was a great role model for young players.
"He instantly commanded respect based on his past accomplishments in the NBA, but earned far more from us with his unpretentious demeanor, his dedication to the job, and his community service in Austin," Khan said. "He was a delight to be around, with a one-of-a-kind laugh that, like him, deserves a spot in the Hall of Fame."
Toros player Anthony Fuqua said teammate Jamar Smith played Johnson in a game of one-on-one after practice and Johnson appeared normal.
"He was being Coach Johnson out there talking trash, playing basketball, dribbling around and shooting," Fuqua said. "Everything seemed fine. We're all in shock."
Toros spokeswoman Perri Travillion said she and Johnson were outside on the sidewalk when he collapsed. He was joking about getting a parking ticket.
"We were laughing," she said. "He just collapsed."
Travillion said she called 911 and that Johnson never regained consciousness. She said Johnson did not appear to have overexerted himself at practice and didn't complain of any discomfort before he collapsed.
Johnson and Bird teamed up for one of the most memorable plays in Celtics history.
During the final seconds of the fifth game of the 1987 Eastern Conference finals against Detroit, Bird stole Isiah Thomas' inbounds pass under Boston's basket and fed Johnson, who drove in for the winning layup. Boston won the series in seven games but lost to the Los Angeles Lakers in the NBA finals.
Bill Laimbeer, the center on that Pistons team, remembered Johnson as a "great player on a great ballclub."
"He played with passion and grit," Laimbeer said. "It was fun to play games like that. You always enjoyed it. It made for not only great games, but great entertainment."
In the 1984 finals, Johnson guarded Magic Johnson effectively in the last four games. In 1985, he hit a last-second jumper against Los Angeles that won the fourth game. In 1986, he was part of a team that featured four Hall of Famers _ Bird, Kevin McHale, Robert Parish and Bill Walton.
"He was truly one of the good guys to play in the NBA, and he was a great teammate who was fun to be around," McHale said.
Johnson had a reputation for deliering in big games.
"I hate to lose," he once said. "I accept it when it comes, but I still hate it. That's the way I am."
He averaged 14.1 points and 5.0 assists for his career. When he retired, he was the 11th player in NBA history to total 15,000 points and 5,000 assists. Johnson made one all-NBA first team and one second team. Six times he made the all-defensive first team, including five consecutive seasons (1979-83).
"As far as a person, he was a great competitor," Sonics teammate Jack Sikma said. "He wouldn't let things pass. He would cause some friction if he felt strongly about something, but with our team that was a good thing."
Former Boston teammate Danny Ainge, now the Celtics' executive director of basketball operations, called Johnson "one of the most underrated players in the history of the game, in my opinion, and one of the greatest Celtic acquisitions of all time."
He said Johnson recently told him how much fun he was having coaching the Toros.
"D.J. was a free spirit and a fun personality who loved to laugh and play the game," Ainge said.
Johnson also was the Los Angeles Clippers' interim coach at the end of the 2002-03 season, going 8-16.
Johnson was born Sept. 18, 1954, in Compton, Calif. He played at Pepperdine and was drafted by Seattle in 1976. Johnson was traded to Phoenix in 1980 and Boston in 1983.
He is survived by his wife, Donna, sons Dwayne and Daniel, and a daughter, Denise.