In 1964, Blassie feuded with Bruno Sammartino and Bobo Brazil. Blassie came into the WWWF with his own world title belt, claiming to be the Pacific World Champion, and was coming to Sammartino's "back yard" to unify the world title. The series began at Roosevelt Stadium, Jersey City, New Jersey in 1964, with Blassie winning on a technicality, but not a pin. The rematches were held at Madison Square Garden in New York, with Sammartino winning out. He returned to the company in 1971 to challenge Pedro Morales for the WWWF Championship. During this stint with the company, Blassie was managed by his future nemesis, "Captain" Lou Albano.
Blassie retired from active wrestling in 1974. Afterwards he became a manager in the World Wide Wrestling Federation (WWWF) and its subsequent incarnation the World Wrestling Federation (WWF). He performed for that promotion until his full retirement from professional wrestling in 1986.
Among the men he managed were Nikolai Volkoff, Blackjack Mulligan, High Chief Peter Maivia, "The Crippler" Ray Stevens, Adrian Adonis, Jesse Ventura, Dick Murdoch, The Iron Sheik, Swede Hanson, Killer Khan, George 'The Animal' Steele, Professor Tanaka, Mr. Fuji, Victor Rivera, Kamala, Hercules Hernandez, and Hulk Hogan. Blassie also managed Muhammad Ali in his boxer vs. wrestler match in 1976 against Antonio Inoki.
One of Blassie's most famous proteges was The Iron Sheik. Blassie led Shiek to the WWF World Heavyweight Championship over Bob Backlund on December 26, 1983 in Madison Square Garden. After Sheik lost the belt to Blassie's former managed wrestler Hulk Hogan, Blassie managed Sheik in various rematches around the country throughout the first half of 1984, culminating into another major televised rematch on December 28, 1984 once again in Madison Square Garden. In this match Hogan was once again victorious. After managing Sheik through a brief feud with Andre the Giant, he also became the manager of Nikolai Volkoff, with Sheik and Volkoff later becoming a tag team. Blassie would gain even more gold when Volkoff and Sheik defeated The US Express for the WWF Tag Team Championship at the first ever WrestleMania. Blassie got involved when he threw his cane into the ring, with Sheik and Volkoff using it to their advantage to gain the victory. When Gene Okerlund confronted Blassie in an interview after the match, Blassie went, "what cane, I didn't have no cane!"
Blassie would continue to manage Sheik and Volkoff even after they lost the tag team titles. In the Summer of 1985, Blassie would start up a house show feud with former friend Lou Albano, culminating into a few matches, including a steel cage match at the Meadowlands in July 1985. Other than these handful of matches, Blassie mostly stayed out of the ring, sticking to managing from ringside. Blassie unsuccessfully campaigned along with the other managers to manage "Macho Man" Randy Savage when Savage first entered the WWF in June 1985. Blassie would continue to interfere from ringside in matches, especially during Volkoff's feud with Corporal Kirchner. Blassie's interference at WrestleMania 2 cost Volkoff his match with Kirchner, when the cane was grabbed by Kirchner instead of Volkoff.
In the Summer of 1986, Blassie started to slowly phase himself out of the WWF and into retirement. On an episode of WWF Championship Wrestling, Blassie barged into the announcer booth to proclaim that he had sold half of the contracts of Sheik and Volkoff to a mysterious benefactor for a large sum of money. This benefactor was later revealed to be Slick. Blassie appeared alongside with Slick to co-manage Sheik and Volkoff for a few television appearances before finally bowing out in November 1986.
Because of his close relationship with Vince McMahon, Sr. and his family, Blassie remained on the WWF roster until the day he died. He would sporadically return to make brief appearances following his retirement in 1986, mostly in produced video packages hyping the "new generation" of wrestling. He was given a commentary tryout in 1986 and 1987 for WWF Wrestling Challenge, calling a few matches with Gorilla Monsoon. He briefly turned face in 1989, making an appearance on Prime Time Wrestling angry with Bobby Heenan over money Heenan hadn't paid him back. Then he returned again in 1990 briefly to do color commentary for Prime Time in April with Vince McMahon, as an emergency fill-in after Tony Schiavone quit, playing a heel once again.
Blassie was inducted into the WWF Hall of Fame in 1994. In 1999, he was shown making the opening narration of WrestleMania XV. In 2000, Blassie made an appearance at the Royal Rumble as a judge in the Miss Rumble 2000 Swimsuit contest. In 2001, Blassie appeared in a special Raw is War segment for the Invasion storyline, wherein he exhorts a gathering of WWF wrestlers to stand tall against The Alliance – and later gets mocked by Alliance leaders Shane and Stephanie McMahon. Because of this, Blassie is one of if not the only man to have worked with all four generations of McMahon family. Blassie also appeared in a segment at the Vengeance pay-per-view event in December 2001, where an "undisputed" champion would be crowned. His final wrestling appearance was May 12, 2003 in Philadelphia on Monday Night Raw, three weeks before his death. The segment featured Blassie, his wife, and Raw GM Eric Bischoff, who was about to allow 3 Minute Warning to attack him, until Stone Cold Steve Austin and The Dudley Boyz stepped in. His final words on WWE television were "D-Von, Get the table", to a monstrous applause.
On June 2, 2003, Blassie passed away.