Considered an out-of-nowhere prospect, Coates was picked in the fifth round of the 1991 NFL Draft by the New England Patriots. His first two years with the Patriots were fairly uneventful; in his rookie year he had ten catches for 95 yards and a two-yard touchdown against the Indianapolis Colts that forced overtime in a 23-17 Patriots win. In his second season he had twenty catches for 171 yards and three touchdowns.
His career changed with the 1993 arrival of quarterbackDrew Bledsoe and legendary head coachBill Parcells to the Patriots franchise. Parcells, known for his reliance on tight ends, frequently used then-rookie quarterback Bledsoe on passes to Coates, and the tight end led the Patriots in receptions in 1993 with 53 catches for 629 yards and eight scores, two of them in New England's season-ending overtime win over Miami.
In 1994, his breakout year, he caught 96 passes, the most ever for a tight end to that point (the record was broken by Tony Gonzalez in 2004), for 1,174 yards receiving (The only time in his career he would gain 1,000 yards in a receiving season) and seven touchdowns. He appeared in his first Pro Bowl and would appear in the next four as well.
In 1996 Coates had 62 catches for 682 yards and nine touchdowns; the most dramatic was against the New York Giants in the final game of the regular season as he caught a twelve-yard pass and bulled through Giants defenders for the game-winning score of a 23-22 New England win. His efforts helped New England to a championship appearance in Super Bowl XXXI. His team lost the game 35-21, but he had a good performance in it, leading the Patriots in receiving with 6 catches for 66 yards and a touchdown. From 1995 to 1998, he caught 84, 62, 66, and 67 passes, respectively, in those 4 seasons.
After the 1999 season, Coates was released by the Patriots, and afterwards played for the Baltimore Ravens, where he climbed the all-time receiving charts, winning Super Bowl XXXV in the process. When Coates was released by the Ravens in the following year, he decided to retire, having become the fourth all-time leading receiver at tight end in NFL history, behind Ozzie Newsome, former teammate Shannon Sharpe, and Kellen Winslow. Coates played in 158 games with 499 receptions for 5,555 yards and 50 touchdowns.
After retiring, Coates returned to Livingstone College, where he was head coach, and also coached in NFL Europe. In 2004 he served an internship with the Dallas Cowboys as an assistant for the tight ends under head coach Bill Parcells. In March 2005, Coates was named the tight ends coach for the Cleveland Browns, replacing Rob Chudzinski, under head coach Romeo Crennel who was the former defensive coordinator of Coates' former team, the New England Patriots.
It was announced on July 7, 2008, that Coates would be inducted into the New England Patriots Hall of Fame.