Havlicek also displayed coolness under pressure and the ability to come through in the clutch. Nothing illustrates this better than a play that has entered the mythology of the NBA. In the last game of the divisional finals in the playoffs in 1965, the Celtics were leading the Philadelphia 76ers by one point. With five seconds left on the clock, the 76ers were bringing the ball inbounds. A basket would most likely win the game for them. Havlicek grabbed the inbounds pass and threw it to a teammate who ran the clock out. Johnny Most, the Boston announcer, went into hysterics: "Havlicek stole the ball! Havlicek stole the ball!" The Celtics went on to win the title that year.
In 1966, the Celtic management was shaken up when Red Auerbach became the team's general manager and Bill Russell was named player-coach. Faced with a team whose offensive punch was suffering from the loss of important stars like Cousy and K.C. Jones, Boston's oncourt leadership was put in Havlicek's hands. Although he remained the sixth man, he was named team captain and told to run the offense. He boosted his own offensive play too, pushing his point average above twenty a game for the first time in his career, where it stayed until the mid-1970s. From 1967 through 1973, Havlicek led the Celtics in assists. With such enhanced performance, Havlicek was as responsible as anyone for the will that drove the Celtics to two more NBA titles under Russell, in 1968 and 1969.
Havlicek's role on the Celtics changed again when veterans Bill Russell and Sam Jones both left the team before the 1969-70 season. He was one of the last links to the already legendary Celtics teams of the early 1960s. "All of a sudden, in one year, I was the old man," Havlicek told the New Yorker's Wind. For the first time in his career as a pro, Havlicek was starting games. He boosted his scoring even more in the early 1970s, reaching an average of 27.5 points per game in the 1971-72 season. He was the team's clutch performer too, frequently being called upon to make the critical play in the last seconds of a close game. Although he was in his thirties, he maintained his remarkable stamina, regularly playing forty plus minutes a game under new coach Tom Heinsohn. Most importantly, Havlicek was expected transmit the style and philosophy of Celtics basketball to new players as the team rebuilt.
Havlicek and his example sometimes seemed to be the only thing left of the Celtics' glory years during the often difficult period of rebuilding during the first half of the 1970s and he was outspokenly critical of new players who did not care as much about winning as he did. The frustrations of this period undoubtedly contributed to his decision not to go into coaching after retiring. He realized his own standards were far higher than those of most players; a difference that could only lead to conflict and dissatisfaction. Despite the lows of the 1970s, however, Havlicek remained true to the Celtic organization and its fans. He turned down a multimillion dollar offer to sign with the American Basketball Association, the rival league to the NBA. By 1974, Boston's rebuilding effort was paying off. Under Havlicek's leadership, the Celtics were NBA champions that year, a title that justly came to be known as the "Havlicek's championship." Players and fans alike recognized that he was the player who regularly made things happen. "A team must have a catalyst, who has the ability to change the pace of a game," center Dave Cowens told Wind of the New Yorker. "John does that for us. He creates confidence because of the way he plays, and you pick it up yourself." Two years later Boston won the championship again.
Havlicek remained remarkably healthy and relatively free of serious injury throughout his career. In 1977, he became, for a time, the all-time leader in NBA games played. Still, he played through injury when he had to. He played the 1969 NBA finals against the Los Angeles Lakers with one eye swollen shut and part of the 1973 semi-finals with a nearly useless right arm, the result of a partial shoulder separation. By 1974, Havlicek was already in his middle thirties. Knowing he would begin losing a step here, a fraction of a second of reaction time there, Havlicek started considering retirement, a step he finally took at the close of the 1978 season.
Havlicek was well-prepared for the day he left the game. He had started working in private business in the off-season in 1964 when he joined the Columbus Ohio firm, International Manufacturing and Marketing Corporation, as a sales representative. By the mid-1970s he had become a vice president there. That same year, he launched John Havlicek All Sports Products, a line of games and sporting goods for camping, baseball, badminton and other sports. In the 1970s he endorsed products ranging from socks, to shaving gear, footwear and prepared foods. Once out of basketball, he and his wife Beth, whom he had married in 1967, settled down in Columbus with their two children. In addition to his business interests, Havlicek worked on occasion as a color commentator for basketball broadcasts.