A tough centre back, Niccolai played for Torres before to join Cagliari Calcio, where he played from 1964 to 1976 and won the first and only Serie A championship in the entire club history in 1970. He ended his career in 1977 for Perugia. Niccolai played 218 Serie A matches, and is still remembered today for his proneness to score own goals, including some very spectacular ones.
Niccolai also took part in the Italian squad at the 1970 FIFA World Cup. This led his Cagliari coach Manlio Scopigno to declare: "I could expect everything from my life, but to see Niccolai in international telecast".
He is currently working with the Italian Football Federation as national team scout.