structural:reformatting, stylistic:style_transfer, structural:summarization
Continued excellence: 1964–1967

Despite winning no majors in 1964 (he had three runner-up finishes), Nicklaus led the PGA Tour money list for the first time in his career by a slim margin of $81.13 over Palmer. At The Open Championship at St Andrews, Nicklaus set a new record for the lowest score in the final 36 holes with 66–68 in high winds (the first time in the championship's history that 70 had been broken in each of the last two rounds). This was not enough, however, to win the event; Nicklaus placed second to Tony Lema. Nicklaus also set a record for the lowest final-round score in the PGA Championship with a 64 (since broken by Brad Faxon in 1995 with a 63), but fell three shots short of champion Bobby Nichols and his record-setting score of 271. In 31 official worldwide events in 1964, Nicklaus achieved six victories, seven runners-up, placed in the top five 21 times, the top-10 21 times, and had only one missed cut. Nicklaus and Palmer also defended their team title at the World Cup in Hawaii, with Nicklaus again finishing as low individual scorer.

When Nicklaus won the Masters Tournament in 1965 and 1966, he became the tournament's first back-to-back winner and the youngest two-time and three-time winner. He broke Ben Hogan 's 72-hole scoring record of 274 in 1953 when he compiled a new aggregate of 271 in the 1965 Masters. This record was tied by Raymond Floyd in 1976 and lasted until Tiger Woods shot 270 in 1997, which was a 72-hole record that was subsequently tied by Jordan Spieth in 2015. When Woods and Spieth shot 270, the scores were achieved with significantly improved golf equipment on essentially the same-length golf course over which Nicklaus and Floyd shot 271. During the 1965 tournament, Nicklaus hit 62 of 72 greens in regulation and had 123 putts, inclusive of just one three-putt green. This was good enough to win by nine shots over Arnold Palmer and Gary Player; this margin of victory was a tournament record that would last for 32 years until Woods won by 12 shots in 1997. The week's performance was highlighted by a third-round 64 that consisted of eight birdies and no bogeys. It was of this round that Nicklaus said, "I had never before and have never since played quite as fine a complete round of golf in a major championship as I did in the third round of the 1965 Masters." This round tied Lloyd Mangrum 's record set at Augusta National in 1940 and remained in place until Nick Price shot 63 during the third round in 1986. It was at this time that Bobby Jones stated Nicklaus played a game with which he was unfamiliar; Jones called Nicklaus's result "the greatest performance in all of golfing history".

After Nicklaus's record performance at Augusta National in 1965, the course underwent some minor changes to make it tougher. These modifications and the difficult weather in the 1966 tournament did not deter Nicklaus. He successfully defended his title with an even-par aggregate of 288, 17 shots higher. He won in an 18-hole playoff over Gay Brewer (Brewer had three-putted the 18th green to force the playoff but he redeemed himself the following year by winning the tournament) and Tommy Jacobs by shooting a two-under-par 70. Nicklaus led the PGA Tour money list again in 1965 by a healthy margin over Tony Lema. Nicklaus and Lema formed the U.S. team for the World Cup, held in Madrid, Spain, but could not defend the title, as South Africa won. In all, Nicklaus competed in 28 official worldwide events in 1965, accumulating five victories, seven runners-up, 19 top-5 finishes, 23 top-10 finishes, and zero missed cuts.

In 1966, Nicklaus also won The Open Championship at Muirfield in Scotland under difficult weather conditions; he used his driver just 17 times because of very heavy rough. This was the only major he had failed to win up to that point. This win made him the youngest player, age 26 (his fifth year on Tour), and the only one after Gene Sarazen, Ben Hogan, and Gary Player (until Tiger Woods at age 24 in 2000, also during his fifth year on Tour) to win all four major championships, now known as the Career Grand Slam. Nicklaus eventually accomplished the double career grand slam in 1971 and the triple career grand slam in 1978, winning all four majors two and three times, respectively. Nicklaus was part of another title for the U.S. in the World Cup in Japan. Nicklaus concluded 1966 playing 22 official worldwide events, with four victories, four runners-up, 14 top-5 finishes, 16 top-10 finishes, and zero missed cuts.

In 1967, Nicklaus won the U.S. Open for a second time. The tournament was held at Baltusrol, and he broke Hogan's 72-hole record by one shot with a 275 when he birdied the par 5 18th hole. During the four rounds, Nicklaus hit 61 of 72 greens in regulation. Nicklaus finished this record win with a dramatic 238-yard 1-iron shot, uphill into a breeze and light rain, to the 72nd green (an approximate 260-yard equivalent) and holing out a 22-foot (6.7 m) birdie putt to close out a final nine of 30 and final round of 65 to shoot 275, four shots better than runner-up Arnold Palmer. Nicklaus and Palmer were the only two players to break par for the week. Sports Illustrated ran a cover photo of Nicklaus throwing his leg high in the air with the headline, "Nicklaus Breaks the Open Record". He also finished runner up in The Open Championship and third in the PGA Championship, one shot out of a playoff between Don January and Don Massengale. In 1967, Nicklaus led the PGA Tour money list for the third time. Later that year, Nicklaus and Palmer teamed up for a 13-shot wire-to-wire World Cup victory in Mexico City. Nicklaus competed in 24 official worldwide events in 1967, with five victories, four runners-up, 14 top-5 finishes, 16 top-10 finishes, and one missed cut. For most of his professional career, Nicklaus employed Angelo Argea as his caddie.