Guarnieri began his youth career with Codogna, and he made his professional debut for Como in 1957, also playing for Internazionale, Bologna, Napoli, and ending his career with Cremonese 1973. He most notably player for the Inter Milan team known as La Grande Inter under manager Helenio Herrera, between 1958 and 1967, briefly returning to the club to make 3 appearances during the 1969–70 season. He was part of Inter's European Cup victories in 1964 and 1965, also winning three Serie A titles, two Intercontinental Cups in 1964 and 1965. Although he was unable to win the Coppa Italia throughout his career, he reached the final during the 1964–65 season, narrowly missing out on a treble with Inter.