Born in Treviglio, in the Province of Bergamo (Lombardy), Facchetti began his career with his hometown club, Trevigliese, as a forward, due to his pace, powerful shot, and technique. He was soon noticed by Helenio Herrera, then manager of Inter, who launched him in Serie A in the late 1960–61 season as an attacking full-back on the left, due to his physique, energy, and tackling ability, in addition to his offensive attributes; he made his club and top-flight debut on 21 May 1961, in a 2–0 away win over Roma. The change of role proved to be an effective choice, and eventually Facchetti developed into one of the most effective defenders in Italian football, forming a notable partnership in defence with fellow Italian full-back Burgnich. Facchetti's innovative playing style as one of the first European overlapping full-backs, combining hard defending with offensive prowess, played a key role in the defensive, yet counter-attacking "catenaccio" system of Herrera's "Grande Inter" side that dominated Italian, European, and World Football in the 60s; whilst conceding few goals defensively, Facchetti was also able to contribute offensively with numerous goals and assists. He held the record for most goals in a single Serie A season by a defender, with 10 goals scored during the 1965–66 season, until it was broken by Marco Materazzi during the 2000–01 season.