After being drafted in the second round of the 1956 NFL Draft out of SMU, Forrest Gregg moved to Green Bay, where he quietly developed one of the most physically imposing careers among offensive linemen in pro football history. At 6-foot-4 and 250 pounds, Gregg combined technical skill, quickness off the line, and sharp mental acuity. He became a cornerstone of Vince Lombardi's offensive line, consistently playing right tackle and shifting inside to guard as needed during games.

Lombardi famously gave Gregg the ultimate compliment a head coach could bestow, calling him "the finest player I ever coached" in his book Run to Daylight. That praise was earned through flawless execution on the gridiron. Gregg was the lead technician behind Green Bay's iconic power sweep, consistently firing off the ball with surgical precision to seal off edge defenders and clear lanes for Paul Hornung and Jim Taylor. His pass protection was equally flawless, giving Bart Starr a pristine pocket to operate in during the franchise's 1960s dynasty's highest-stakes moments.

Beyond his elite technique, Gregg stood among the ultimate ironmen in professional sports history. From his rookie season in 1956 through the 1970 campaign, he appeared in a then-league-record 188 consecutive regular-season games. His relentless work ethic and durability produced a remarkable run of individual accolades; between 1959 and 1968, he earned nine Pro Bowl nods and was named First-Team All-Pro seven times (1960, 1962–1967).

Gregg's consistent role on the right side of the line was crucial to Green Bay's five NFL Championship wins (1961, 1962, 1965, 1966, 1967), including Super Bowls I and II. After 14 successful seasons with the Packers, he finished his playing career with the Dallas Cowboys in 1971, earning a sixth ring in Super Bowl VI. He returned to the team in 1984, coaching for four seasons as Green Bay's head coach.

With Green Bay, Gregg played in 193 games, starting 156. He was part of the NFL 1960s All-Decade Team and the NFL's 75th and 100th Anniversary All-Time Teams, and was inducted into the Green Bay Packers Hall of Fame in 1977. That year, he was also enshrined in the Pro Football Hall of Fame.