A former centre back, Mazzone played several seasons for his beloved team, A.S. Roma, as well as for Spal and Ascoli. He spent nine seasons with Ascoli, retiring during the 1968–69 season to become the club's manager, in Serie C, helping the team to win the Serie C title in 1972.
Already popular with the Ascoli fans because of his history as a former player for the club, Mazzone gained even more popularity by leading the team for twelve years, up to their historic first ever Serie A appearance. Successively, Mazzone coached several Serie A and Serie B teams, such as Fiorentina, achieving his personal best result in Serie A, a third-place finish during the 1976–77 Serie A season, also winning the Anglo-Italian League Cup in 1975. He subsequently coached Catanzaro, Bologna F.C. 1909, Lecce, Pescara, and Cagliari (1991–93), leading the team to its first appearance in a European competition since the times of Gigi Riva.
After his successes at Cagliari, Mazzone fulfilled his dream, being called to manage his hometown club AS Roma for the 1993–94 Serie A season, where he coached a young Francesco Totti. He remained at the helm of AS Roma for three seasons, without being able to obtain any notable triumph, however.
In 2000, he took charge of Serie A newcomers Brescia in 2000, where he had the opportunity to coach Roberto Baggio, and subsequently also Andrea Pirlo, Luca Toni, and Pep Guardiola. During his first season with the club, he helped the club to avoid relegation for the first time in 40 years, leading them to their best ever Serie A finish to qualify for the 2001 UEFA Intertoto Cup, where they reached the final. The following season, Mazzone was at the centre of controversy, however, when, on 30 September 2001, during a league match against rivals Atalanta, he ran for about 70 meters and screamed towards the Atalanta supporters, guilty of having offended him for the entire match long with personal insults, after his team tied the score in the final minutes. That season, Brescia once again managed to avoid relegation, and also reached the semi-finals of the Coppa Italia, their best ever result in the competition. In 2002, Mazzone was awarded the Panchina d'Oro award, in honour of his career.
After helping the team avoid relegation for the third consecutive season, Mazzone left Brescia in 2003 in order to coach Bologna for the third time in his long career. However, his adventure lasted only two seasons, as the team was surprisingly relegated to Serie B at the end of the 2004–05 Serie A season. It was the first relegation ever in Mazzone's career, and it convinced him to take a break from coaching for a while.
However, on 7 February 2006, at the age of 68 years and 11 months, Mazzone accepted an offer of Livorno, filling the coaching position which had been left vacant by Roberto Donadoni, who resigned the day before; he resigned at the end of the season.
Mazzone is the most experienced coach in the Italian football panorama, having coached on more than 1,000 professional matches. On 18 March 2006, Mazzone has become the Italian manager who coached the highest number of Serie A matches ever, equalling and then overcoming the past record held by Nereo Rocco.
Roberto Baggio often defined several times Mazzone as one of the best coaches he met in his playing career. Baggio himself, in a radio interview on 18 February 2006, declared that Mazzone asked him to join him in a playing football comeback in Livorno; however, Baggio claimed that, even if he would have given him an affirmative answer because of his gratitude for what Mazzone did for him, he was forced to refuse because of his physical troubles.
Pep Guardiola said that he learnt a lot from Mazzone during his Brescia years.
He is often nicknamed Sor Carletto because of his Roman origins, and the strong accent typical of the inhabitants of the Italian capital city.
Despite having coached around the whole country for 35 years and more, Mazzone has always held his residence in Ascoli Piceno, home of Ascoli Calcio, the team in which he started his long managing career.