Valoti was born in Vicenza on 6 September 1993, the son of former footballer Aladino Valoti, who played for several Serie A and Serie B teams during the late 1980s and 1990s and was the director of football at Albinoleffe.
Mattia started playing football at the age of 5 in the youth team of Lucchese, the club where his father was playing at the time. Over the next eight years, Mattia played for several clubs, namely Cosenza, Palermo, Crotone, Martina Franca and Dalmine Futura, as a result of his father's transfers.
In 2006, Valoti settled at AlbinoLeffe, spending five seasons in the club's youth system. He went on to make his professional debut, aged 17, coming on as a substitute in a Coppa Italia game against Crotone, on 27 October 2010. Three days later, he also made his Serie B debut, again coming off the bench in a match against Novara.
During the January 2011 transfer window, Serie A club Milan signed Valoti to a co-ownership deal, for €800,000, though they allowed Albinoleffe to keep him on loan for the remainder of the season.
At the beginning of the 2011–12 campaign, the co-ownership deal was renewed and Valoti joined Milan, taking part in the pre-season trainings and friendlies with the first team. However, he did not make any official appearance during his first season with the Rossoneri and instead he played mainly with the Primavera (under-20) team.
For the 2012–13 season, the co-ownership was extended again. However, Valoti still made no appearances for the first team (instead featuring as an overage player for the Primavera team, which is now an under-19 team) and during the January transfer window he was loaned out to his former club Albinoleffe.
Valoti returned to AlbinoLeffe in temporary deal on 24 January 2013. In June 2013 the co-ownership was renewed. On 25 July Milan extended the temporary deal of Valoti at AlbinoLeffe. On 20 June 2014 Milan sold Valoti back to AlbinoLeffe for €400,000 fee. However, AlbinoLeffe decided to sell Valoti again on 15 July.
On 15 July 2014 Serie A club Verona signed Valoti in a temporary deal, with an option to buy. His permanent transfer was confirmed on 7 July 2015, after AlbinoLeffe had relegated from professional leagues.
On 23 July 2015 Valoti was signed by Pescara (loan) along with Pierluigi Cappelluzzo (loan) and Francesco Zampano (definitive).
On 1 February 2016 Valoti was signed by Livorno.
As of 15 July 2015.
1European competitions include the UEFA Champions League.A.C. Milan
Supercoppa Italiana: 2011
2Other tournaments include the Supercoppa Italiana and Serie B relegation play–offs.