Schwarzenegger served in the Austrian Army in 1965 to fulfill the one year of service required at the time of all 18-year-old Austrian males.[4][15] During his army service, he won the Junior Mr. Europe contest. He went AWOL during basic training so he could take part in the competition and spent a week in military prison: "Participating in the competition meant so much to me that I didn't carefully think through the consequences." He won another bodybuilding contest in Graz, at Steirer Hof Hotel (where he had placed second). He was voted best built man of Europe, which made him famous.
"The Mr. Universe title was my ticket to America – the land of opportunity, where I could become a star and get rich."[16] Schwarzenegger made his first plane trip in 1966, attending the NABBA Mr. Universe competition in London.[15] He would come in second in the Mr. Universe competition, not having the muscle definition of American winner Chester Yorton.[15]

Charles "Wag" Bennett, one of the judges at the 1966 competition, was impressed with Schwarzenegger and he offered to coach him. As Schwarzenegger had little money, Bennett invited him to stay in his crowded family home above one of his two gyms in Forest Gate, London, England. Yorton's leg definition had been judged superior, and Schwarzenegger, under a training program devised by Bennett, concentrated on improving the muscle definition and power in his legs. Staying in the East End of London helped Schwarzenegger improve his rudimentary grasp of the English language.[20][21] Also in 1966, Schwarzenegger had the opportunity to meet childhood idol Reg Park, who became his friend and mentor.[22] The training paid off and, in 1967, Schwarzenegger won the title for the first time, becoming the youngest ever Mr. Universe at the age of 20.[15] He would go on to win the title a further three times.[14] Schwarzenegger then flew back to Munich, training for four to six hours daily, attending business school and working in a health club (Rolf Putzinger's gym where he worked and trained from 1966–1968), returning in 1968 to London to win his next Mr. Universe title.[15] He frequently told Roger C. Field, a friend in Munich at that time, "I'm going to become the greatest actor!"

Schwarzenegger is considered among the most important figures in the history of bodybuilding, and his legacy is commemorated in the Arnold Classic annual bodybuilding competition. Schwarzenegger has remained a prominent face in the bodybuilding sport long after his retirement, in part because of his ownership of gyms and fitness magazines. He has presided over numerous contests and awards shows.

For many years, he wrote a monthly column for the bodybuilding magazines Muscle & Fitness and Flex. Shortly after being elected Governor, he was appointed executive editor of both magazines, in a largely symbolic capacity. The magazines agreed to donate $250,000 a year to the Governor's various physical fitness initiatives. The magazine MuscleMag International has a monthly two-page article on him, and refers to him as "The King".

One of the first competitions he won was the Junior Mr. Europe contest in 1965.[4] He won Mr. Europe the following year, at age 19.[4][15] He would go on to compete in, and win, many bodybuilding contests. His bodybulding victories included five Mr. Universe (4 – NABBA [England], 1 – IFBB [USA]) wins, and seven Mr. Olympia wins, a record which would stand until Lee Haney won his eighth consecutive Mr. Olympia title in 1991.