He won the German championship with 1. FC Köln in 1962, and was awarded the (German Footballer of the Year), performed superb in the World Cup and was subsequently named in the World Cup Dream-Team 11. His debut in the Serie A came in 1963, when Schnellinger left 1. FC Köln for A.C. Mantova, in a match against A.C. Milan which ended in a surprising 4–1 victory for A.C. Mantova. However he played there only for one season, and was signed by A.S. Roma in 1964 when they won the Coppa Italia, and finally by A.C. Milan in 1965. Schnellinger was bought up together with Roma teammate Angelillo and Sormani. He played nine seasons with the Rossoneri, obtaining several successes both at the national and European level. He was one of the first successful German footballers abroad.
He participated in his first 1958 World Cup in Sweden at the young age of 19, and went on to become one of the few players to play in 4 World Cups (1958, 1962, 1966, 1970). He was renowned for his physical power, pace, and his winning mentality. His only international goal came in the last minute to draw 1–1 in the thrilling semi-final of the 1970 World Cup against Italy which later became known as the "Game of the Century". Italy eventually won 4–3 after extra time. He won his last cap in 1971.