In the Chiefs' first two seasons attendance did not match the levels Mayor Bartle had promised, but in 1966 average home attendance at Chiefs games increased and reached 37,000. By 1969 Chiefs' average home attendance had reached 51,000. In 1966 the Chiefs won their first AFL Championship (after having previously won it as the Dallas Texans) and reached the first-ever Super Bowl, which the Chiefs lost to the Green Bay Packers. The Chiefs remained successful through the 1960s, and in 1970 the Chiefs won the AFL Championship and Super Bowl IV (the last Super Bowl played when the AFL was a separate league prior to it being absorbed into the NFL as the American Football Conference) over the heavily favored Minnesota Vikings.
In 1966, the NFL and AFL agreed to merge, with a championship game between the two leagues to be played after that season. In a July 25, 1966, letter to NFL commissioner Pete Rozelle, Hunt wrote, "I have kiddingly called it the 'Super Bowl,' which obviously can be improved upon." Hunt would later say the name was likely in his head because his children had been playing with a Super Ball toy. Although the leagues' owners decided on the name "AFL-NFL Championship Game", the media immediately picked up on Hunt's "Super Bowl" name, which would become official beginning with the third annual game, which was won by the AFL's New York Jets over the NFL's Baltimore Colts.