Born in Chicago and a graduate of Carver High School, Cummings attended DePaul University from 1979 to 1982. He averaged 16.4 points per game over 85 games and entered the 1982 NBA draft after departing from school.
He was selected in the first round by the San Diego Clippers, and in his inaugural 1982–83 season, he won the NBA Rookie of the Year Award after putting up 23.7 points and 10.4 rebounds per game. These figures would turn out to be the highest of his career in those categories. Late in his rookie season, Cummings suffered from heartbeat irregularities, which would keep him out the remaining two weeks of the season. The team lost every game without him.
After the next season (1983–84), he was traded to the Milwaukee Bucks, where he would continue to post above 20 point and 8 rebound averages for four out of his five years on the team. As a Buck, Cummings was selected to play in the 1984–85 and 1988–89 NBA All-Star Games.
He was traded to the San Antonio Spurs where he would remain for six years. His scoring and rebounding averages for the 1989–90 through 1991–92 seasons were close to 20 and 8, respectively, and he helped his team to consecutive 50-win seasons and playoff appearances. By this time, he was recognized as a reliable power forward in the league.
Cummings suffered a serious knee injury in the summer of 1992 in a casual pickup game. He would miss the first 74 regular season games. Upon his return to the lineup, he could no longer put up near-All-Star numbers, and from then on he was used in a more suitable role as a reserve. Cummings would play in San Antonio until 1994–95.
In November 1995, he joined the Milwaukee Bucks again. He played in 81 games averaging 8.0 points per game in 21 minutes of work.
In January 1997 he signed with the Seattle SuperSonics as a free agent. He contributed as a role player helping the Sonics reach the Western Conference Semi-finals where they lost to the Houston Rockets in 7 games.
He signed with the 76ers in September 1997. He played in 44 games averaging 5.3 points per game.
Just before the trade deadline in February 1998, he was traded to the New York Knicks for Herb Williams and Ron Grandison. For New York he played in 30 games to finish the 1997-98 season and a total of 74 games combined between Philadelphia and New York.
Prior to the lockout ending in 1999, he was traded by the Knicks along with John Starks and Chris Mills to the Golden State Warriors for Latrell Sprewell. He managed to play 2 seasons for the Warriors, then retired from basketball after the 1999-2000 season.
In 18 seasons Terry Cummings scored 19,460 points, falling just short of the 20,000 point mark, but placing him among the top 50 career scorers. He finished with averages of 16.4 points per game and 7.3 rebounds per game. He also played in 1,183 Games, had 33,898 minutes, a .484 field goal percentage (8,045 for 16,628), .706 free throw percentage (3,326 for 4,711), 8,630 total rebounds (3,183 offensive, 5,447 defensive), and 1,255 steals.
Cummings has been an ordained Pentecostal Minister since 1977 and performed service at the wedding of former teammate Sean Elliott. He has three sons, Antonio, T. J., and Shawn.
In a creative turn of his career, Cummings released an album, T.C. Finally in early 2007, of songs which he wrote, sang, and played keyboards. The album is reminiscent of the R&B/soulstyles of musicians such as Marvin Gaye, Al Green, and Sam Cooke.