A graduate of North Texas State University (now the University of North Texas) in 1960 he chose to play for the Dallas Texans and led the American Football League in rushing attempts, yards, and TDs in the league’s first year. Haynes helped launch the AFL in 1960, when he was the fledgling league's first Player of the Year , and its first Rookie of the Year. He captured the AFL's first rushing crown with 875 yards, and also led the Texans in receiving, punt returns, and kickoff returns. Haynes spent three years in Dallas and two with same franchise when it became the Kansas City Chiefs.

Haynes still owns 11 franchise records, including most points in a game (30), most touchdowns in a season (19), most touchdowns in a game (5), most career 100-yard rushing games (12), most career rushing touchdowns (39) and most career combined yards (8,442). Over his career he was regularly among the American Football League's top ten rushers, ranking third all-time, and the all-time leader in touchdowns, with 46. He was Hall of Fame head coach Hank Stram's most versatile and dangerous weapon from 1960-62, amassing 43 touchdowns and 4,472 yards on rushes and receptions. In 1962, he helped the Texans win the American Football League championship in the classic double-overtime victory over the defending champion Houston Oilers. At the time it was the longest professional football championship games ever played. In that game, Haynes scored touchdowns on a 28-yard pass reception from quarterback Len Dawson, and on a 2-yard run.

"He was a franchise player before they talked about franchise players," praised Stram. "He did it all-rushing, receiving, kickoff returns, punt returns. He gave us the dimension we needed to be a good team in Dallas."

The 6-foot-1, 200 pound (91 kg) Haynes, who had great speed and dazzling moves in the open field, set AFL records with 5 touchdowns in a game and 19 touchdowns in a season in 1961, and with 46 career rushing touchdowns. He also played for the Denver Broncos, the Miami Dolphins, and the New York Jets.

During his 8 professional seasons, Haynes carried the ball 1,036 times for 4,630 yards, a 4.5 average; caught 287 passes for 3,535 yards, a 12.3 average, and 20 touchdowns; returned 85 punts for 875 yards, a 10.3 average, and 1 touchdown; and ran back 121 kickoffs for 3,025 yards, a 25.0 average, and 1 touchdown. His 12,065 combined yards is the American Football League record. Haynes had three games in which he gained 100 or more yards on 14 or fewer carries, and was selected to the All-Time All-AFL second team. He has a program called Heroes of Football which connects former professional players with their communities.