Mangia started his coaching career at age 30, working alongside Sean Sogliano at Eccellenza club Varese and leading them to two promotion, and back into professionalism, in three years. He successively worked at Serie D and Serie C2 clubs from the Northern Italy such as Tritium, Ivrea and Valenzana. In 2010 he returned at Varese, this time as youth coach, and guided a team composed by unknown youngsters to a spot in the Campionato Primavera final.
In June 2011, Mangia agreed to follow director of football Sean Sogliano at Palermo, accepting a job offer as head of the under-19 team at the Sicilian club in place of Paolo Beruatto.
Later on 31 August, Mangia was surprisingly appointed as temporary head coach following the dismissal of Stefano Pioli, under recommendation by Sogliano; despite not having the required coaching badges, he was allowed to serve as head coach due to having been admitted to the national 2011–12 UEFA Pro license course. His first game in charge of team duties, played on 11 September against European powerhouse Inter, saw Palermo, now switched to a brand-new 4–4–2 tactical disposition, achieving an astonishing 4–3 win thanks to a brace from captain Fabrizio Miccoli. Under Mangia's tenure, Palermo significantly improved and achieved three wins and 10 points in the first five Serie A games, including a consecutive two games with a clean sheet, a relevant feat for what was the second-worst defence in the previous season; such results, together with the strict relationship between Mangia and director of football Sean Sogliano and Delio Rossi's confirmed refusal of a Palermo comeback, led the press to speculate about the possibility of him being offered a new contract as permanent head coach. Despite Sogliano's resignation as director of football on 2 November 2011, Mangia was then eventually offered a two-year contract as head coach, which he signed on 4 November 2011. On 19 December 2011, he was sacked from his position after the 2-0 derby defeat to Sicilian rivals, Catania Calcio the day before, along with assistant coach Onofrio Barone.
Devis Mangia has been hired as Italy's under-21 coach, replacing Ciro Ferrara. Mangia described the job as "one of the most prestigious in Italian football" and says "it is a sign of great esteem and responsibility, I'm really happy". Mangia has vowed to follow the model of Italy's senior team coach Cesare Prandelli, who guided the Azzurri to the 2012 UEFA European Football Championship final.