In 1966, Arion debuted in the World Wide Wrestling Federation. Arion quickly became a major star, defeating top opponents, including Dr. Bill Miller, Cowboy Rocky Fitzpatrick, Gorilla Monsoon, Johnny Valentine, Crazy Luke Graham, Bull Ortega, The Sheik, Kentucky Butcher, and George Steele. In his second match in the U.S., he teamed with Tony Parisi to defeat Baron Mikel Scicluna and Smasher Sloan for the WWWF United States Tag Team Championship. Parisi and Arion won the titles, and were never defeated for the belts. In June 1967, Parisi gave his half to Arnold Skaaland, marking his second United States Tag Team Title reign. On July 10, 1967, they lost the titles to the Sicilians (Lou Albano and Tony Altomare). He became a protégé of WWWF World Heavyweight Champion Bruno Sammartino, and they both beat up the Sicilians, winning in two straight falls on July 24 to become the final U.S. Tag Team Champions.
Sammartino and Arion teamed up to main event at Madison Square Garden twice against Gorilla Monsoon and Professor Toru Tanaka. Monsoon and Tanaka won the first match on disqualification; Arion and Sammartino won a decisive victory in a "Texas Death Match" rematch, two falls to one.
Arion returned to the WWWF in 1974 and was instantly a fan favorite. His popularity increased when he teamed with Chief Jay Strongbow. Before Arion and Strongbow took on the Valiant Brothers, there were signs that this tag team would not last. While Arion was in a singles match with Killer Kowalski, the latter had the upper hand. Strongbow came to Arion's assistance, but the Greek was enraged because Strongbow's outside interference cost him the match via disqualification. Even though it was a disqualification, Arion complained that it was his first loss of any kind in the U.S.. In their match against the Valiant Brothers, Arion suddenly turned on his partner. He quickly went from fan favorite to heel when the fans learned that Freddie Blassie now took him under his management.
Arion signed for a match against Bruno Sammartino's protégé, Larry Zbyszko at Madison Square Garden. During an interview, the announcer wanted to know why Arion signed for the match—it was deemed a mismatch. Arion laughed, and said that "Larry is my friend". Arion demolished Zbyszko in the Garden, and Bruno Sammartino ran in at the end, after the pin, to prevent further harm by Arion.
Arion then feuded with Strongbow, pinning him in numerous arenas; and went on to a three bout series at Madison Square Garden against his former partner, WWWF Champion Sammartino, whom he blamed for losing the tag team titles at the expense of his singles title.
In early 1976, Arion left the WWWF, and returned in late 1977. In 1978, he partnered with Freddie Blassie's newest turncoat, the once popular Victor Rivera.