Joiner graduated from Grambling State University in 1969 and was drafted in the fourth round by the American Football League's Houston Oilers. He started his career as a defensive back, but he made the switch to wide receiver in his rookie year after being carted off the field from a hit by Denver Broncos running back Floyd Little. Joiner played for Houston until 1972, when he was traded to Cincinnati. From 1972 to 1975 he played for the Cincinnati Bengals. In 1975 he was traded to the San Diego Chargers, with whom he remained for eleven seasons before retiring as a player after the 1986 season. Before leaving the Bengals, he set a franchise record with 200 receiving yards in a single game.
It was with the Chargers' high flying "Air Coryell" offense under coach Don Coryell that Joiner had his most productive years, exceeding 1,000 yards receiving in a season four times and going to three Pro Bowls (1976, 1979–80). Joiner was selected All-Pro in 1980 and 2nd Team All-AFC in 1976. Although he never played in a Super Bowl, Joiner helped the Chargers reach the AFC title game in the 1980 and 1981 seasons. In the 1980 AFC championship game, he caught 6 passes for 130 yards and 2 touchdowns. In January 1982, he played a key role in San Diego's 41-38 divisional postseason overtime win over the Miami Dolphins in a game that is known in NFL Lore as The Epic In Miami. Joiner caught 7 passes for 108 yards in the game, including 2 key receptions on his team's game winning drive in overtime. His 29-yard reception on the penultimate play of the game set up the winning field goal.
Joiner finished his 18 AFL/NFL seasons with 750 receptions for 12,146 yards and 65 touchdowns. He retired as the then-NFL leader in career receptions, yards, and games played by a wide receiver (239). Joiner credited his success and longevity to Coryell: "Thanks to Coach Coryell’s offense and his revolutionary passing game, he prolonged my career, from the day I got to the Chargers until the day I retired. I will forever be grateful to him and what he did for the game of football.”
In addition to good health and longevity, Joiner was an intelligent player and precise pass route runner. Hall of Fame coach Bill Walsh called Joiner "the most intelligent, the smartest, the most calculating receiver the game has ever known." Following his playing career, Joiner successfully transitioned into a receiver's coach with the Kansas City Chiefs and the San Diego Chargers. Joiner was the last former American Football League player (Oilers, 1969) to retire from Professional Football, in 1986.
In 1999, he was ranked number 100 on The Sporting News' list of the 100 Greatest Football Players.