DiGregorio played on the 1968 Rhode Island (Class B) champions at North Providence High School.
DiGregorio and Marvin Barnes lead Coach Dave Gavitt's Providence team to a Final Four appearance in the NCAA Tournament in DiGregorio's senior season, where they eventually lost to Memphis State, but only after Barnes sustained a knee injury that forced an early exit. After playing for the Providence College Friars, DiGregorio played on a college all-star team, and along with Bill Walton, led the U.S. in defeating a Soviet team in an exhibition game which helped heal the still-open wound of the United States' loss in the 1972 Summer Olympic finals debacle. Ernie "D" was then drafted by the Kentucky Colonels of the American Basketball Association but opted instead for the NBA.
He was selected third overall by the Buffalo Braves in the 1973 NBA Draft out of Providence College, and won the NBA Rookie of the Year Award in 1973-74 after averaging 15.2 points and leading the league in both free throw percentage and assists per game. DiGregorio still holds the NBA rookie record for assists in a single game with 25 (a record now shared with Nate McMillan). He would never again come close to that level of production, but managed to have a decent NBA career, most of which he spent with the Braves.
After playing 27 games with the Braves in the 1977-78 season, DiGregorio was traded to the Los Angeles Lakers, but he played in a Lakers uniform in only 25 games before being waived. The Boston Celtics signed him as a free agent but he played only sparingly for the rest of the season. He would not play in the NBA again, although he did not formally retire until 1981.