The playful career of Mario Beretta was only of short duration. He played only at the AC Pro Sesto.
In 1982, he received at the age of 23 years, his first job as a youth coach at the club amateur ASD Abbiategrasso. The following 12 years, he was still working at AC Monza Brianza and Como Calcio in the same function. Besides, he was a teacher at the Istituto Leopardi in Milan. In 1994, Beretta was then his first job as coach of a men's team, here the amateur club U.S. Corsico. A year later he took over the team of his former clubs Pro Patria Calcio, which is now played in the lowest Italian professional league, the C series. There were other coach stations at FBC Saronno, Como Calcio and AC Lumezzane before he hired the Varese FC. With this team Beretta reached the play-offs for the Serie B, but failed in the semifinals to AS Cittadella. He was hired as manager of Ternana Calcio 2002, which was his first job in the B series equivalent.
Two years later he joined Chievo Verona in Serie A heavy succession of success coach Luigi Delneri to, who had once led the team after the climb season in the UEFA Cup. After a perfect start under Beretta Verona was meanwhile climbed up to third place in the table behind Juventus and AC Milan in the 2004/05 season, but gradually the performance of the team deteriorated constant. With three games left in the season, the team was only third last and Beretta was replaced by his former assistant coach Maurizio D' Angelo. In the following season he coached the FC Parma and reached the tenth place. Through football scandal in Italy in 2005/ 06, however, Parma moved up to seventh place and thus qualified for the UEFA Cup. Nevertheless, Beretta left the club and went to Siena, which he put over the entire 2006/07 season in a relegation battle. Despite number 14 and the associated relegation he was released on end of the season, and was succeeded by Andrea Mandorlini.
Just five months after his release Beretta was reinstated as a coach in Siena on November 12, 2007. Despite the renewed successful relegation he did not renew his contract and joined the 2008/09 season to Erstligaaufsteiger U.S. Lecce. On 9 March 2009 he was released before the end of the season again, as Lecce was only on the 19th place in the table after last 6 games without a win. From 29 November 2009 to 10 January 2010, Mario Beretta was coach of FC Turin. Already after 4 games ( 3 losses, 1 draw ), he was curiously replaced by his predecessor Stefano Colantuono again. In June 2010 he was appointed head coach of the Greek first division club PAOK Thessaloniki. On July 22 it was announced that he has to leave his post after just one month. The club was unhappy with his pre-season.
On 6 December 2010 he was introduced as head coach at Brescia Calcio.