Cappelletti was out of pro football in 1959, back in Minnesota. With the launch of the American Football League in 1960, he joined the Boston Patriots and was initially a kicker and defensive back. He switched to offense late in that season and teamed up with quarterback Babe Parilli to form a tandem nicknamed "Grand Opera Twins", due to their Italian surnames. Cappelletti won AFL MVP honors in 1964, led the league in scoring five times and was a five-time AFL All-Star. He holds the professional football record for points over a six-year period (9.5), points over an 11-year period (7.5) and percentage of his team's total points over an eight-year period (34%). One of 20 AFL players active during the entirety of the league's ten-year existence, Cappelletti was also among just three players who played in every one of his team's AFL games. He played with the Patriots all 11 years in Boston, from 1960 through the 1970 NFL merger season, and retired in late August 1971 at age 37; he was the AFL's all-time leading scorer with 1,130 points (42 TDs, 176 FGs and 342 PATs) and among the AFL's top ten all-time receivers in yards and in receptions. Cappelletti had two of the top five scoring seasons in pro football history, with 155 points in 1964 and 147 points in 1961 (14-game seasons). His Patriots team scoring record lasted until it was broken by Adam Vinatieri on December 5, 2005. To date, as of the end of the 2017 season, Cappelletti is the Patriots' 11th all-time leading receiver in receptions with 292 catches and 9th in receiving yards with 4,589 yards. He is 5th in Patriots history in receiving touchdowns with 42 and has the most field goal attempts (334) in team history.