After appearing in only two games during his rookie season, he was traded to the Philadelphia Eagles and quickly emerged as one of the top receivers in the NFL, finishing the 1969 season with 65 receptions for a league-best 1,116 yards and 9 touchdowns. During his years with the Eagles, Jackson led the NFL in receptions in 1972 and in receiving yardage in both 1969 and 1972.
Prior to the 1973 NFL season, the Eagles traded Jackson back to the Rams for quarterback Roman Gabriel (who was expendable as the Rams had acquired John Hadl). Jackson led the NFL in receiving touchdowns in 1973 with 13 and helped the Rams rebound from a 6–7–1 record the previous season to a 12–2 finish that won them the first of 7 straight NFC West Division Championships. Statistically, his greatest NFL game came against Dallas on October 14, 1973 when he caught 7 passes for 238 yards and 4 TDs (this came in the midst of a string of four games in which he caught a total of 13 passes for 422 yards for a 32.5-yard average with 8 TDs). During his career, Jackson was selected to play in the Pro Bowl 5 times. In 1972, he was named 2nd-team All-Pro by the Associated Press (AP), the Pro Football Writers Association (PFWA) and the Newspaper Enterprise Association (NEA) and was voted 1st-team All-NFC by the Associated Press (AP), The Sporting News and UPI. He was a consensus 1st-team All-Pro and All-NFC choice in 1973. He was voted 2nd team All-NFC by UPI in 1976, 1st team All-NFC by The Sporting News and UPI in 1977 and 2nd team All-Pro by the NEA in 1977.
In 1978, Jackson was traded to the New England Patriots. Alongside Stanley Morgan, Jackson helped formed one of the NFL's best starting wide receiver duos of the late 1970s/early 1980s. In 1979, Jackson finished with 1,013 yards receiving, 7 touchdown receptions and was second in the NFL in yards-per-catch with an average of 22.5. His teammate Morgan, who was nine years younger than Jackson, was the only one to finish with a higher average that season. Jackson finished out his career by playing one season each for the Minnesota Vikings (1982) and the Seattle Seahawks (1983).
During his career, Jackson had 29 career 100-yard games and three 1,000-yard seasons. At the time of his retirement, only Don Maynard had more career receiving yards than Jackson. He currently ranks 26th in league history in career receiving yardage. For the decade of the 1970s, Jackson ranked first in receptions (432), yards (7,724) and receiving touchdowns (61). Despite this, Jackson was not one of the wide receivers selected to the NFL's All-Decade Team for the 1970s and he has yet to be elected to the Pro Football Hall of Fame.