A native of Tuscaloosa, Alabama, Stallworth was an All-Southern Intercollegiate Athletic Conference receiver for Alabama A&M in 1972 and 1973. Stallworth was the 82nd player taken that year. After a rookie year as an understudy, he became a starter in his second season and held that job for the rest of his 165-game career. Stallworth did battle a series of fibula, foot, ankle, knee and hamstring injuries that forced him to miss 44 regular-season games.

He is best remembered for his actions in the Super Bowl. In Super Bowl XIII, he caught a record-tying 75-yard touchdown pass from Terry Bradshaw that would later be a crucial touchdown in a 35-31 Steelers win over the Dallas Cowboys. He suffered leg cramps later and played sparingly in the second half, finishing with 3 receptions for 115 yards and 2 touchdowns

One year later, at Super Bowl XIV with the Steelers trailing the Los Angeles Rams 19-17 early in the fourth quarter, Steelers' coach Chuck Noll called for "60-Prevent-Slot-Hook-And-Go," a play the Steelers failed in practice prior to the big game. With 12 minutes remaining, Bradshaw dropped back and threw it long to Stallworth, who caught it and beat Rod Perry to the end zone for a 73-yard touchdown that would pave the way for the Steelers' 31-19 win and their fourth world championship. Sports Illustrated considered the catch notable enough to put Stallworth on the cover of a subsequent issue. Overall, Stallworth recorded 3 receptions for 121 yards in the game.

John Stallworth holds the Super Bowl records for career average per catch (24.4 yards) and single-game average, 40.33 yards in Super Bowl XIV. He had 12 touchdown receptions and a string of 17 straight games with a reception in post-season play. Stallworth also scored touchdowns in eight straight playoff games at one point (1978-1983), an NFL record.

He led the AFC with a career-high 1,395 yards gained on 80 receptions in 1984, when he was named the NFL Comeback Player of the Year. He helped the Steelers defeat eventual Super Bowl champion San Francisco for that team's only loss of the season and led the Steelers in a playoff run that featured an upset win over the Denver Broncos in the AFC Divisional Playoffs at Denver's Mile High Stadium. The next week, the Steelers fell lost to the Miami Dolphins in the AFC Championship Game, thus falling short of participating in Super Bowl XIX.

In 1986, he founded Madison Research Corporation (MRC), which specialized in providing engineering and information technology services to government and commercial clients. Under Stallworth's leadership, MRC grew to more than 650 employees and $69.5 million in revenues.

Stallworth was inducted into the Pro Football Hall of Fame in August 2002.

John Stallworth was announced as becoming part-owner of his former team on March 23, 2009, as part of the Rooney family restructuring ownership of the team. The Rooneys will remain in control of the franchise. Stallworth joins former Pittsburgh Penguins captain Mario Lemieux among Pittsburgh sports figures that would later own at least part of their former team.