A provincial painter, Moroni seems to have devoted his talents principally to portraiture. According to Moroni’s 1st biographer, Carlo Ridolfi, writing in 1648, Moroni’s skill as a portraitist was highly esteemed by his contemporaries. The painter Titian supposedly recommended to Venetian officials traveling to Bergamo that they have their portraits painted by Moroni because of the naturalism of his style. He also received commissions for religious paintings.
Following the example of his older brother, Ettore, Mario enlisted in military service under the Holy Roman Emperor, Charles V. Mario fought against both the Protestant princes of the Schmalkaldic League in Germany in the battle of Muhlberg (1547), and the French troops in the war in Piedmont (1551-1555). In 1556, Charles V abdicated; Mario, who had attained the rank of captain, probably gave up his military career at that time.
His family, among the richest & most influential in Crema, was also notorious for acts of violence & evil deeds. In 1537, e.g., brother Ettore killed 2 adversaries in one year; at the same time, their father, Agostino, was imprisoned in a Venetian jail. Ettore died a violent death in 1574, as did his son Maurizio, whose life had been equally wicked. It is not surprising, therefore, that Mario preferred to retire at first to Crema, rather than Bergamo where the family reputation was not good.
However, documents show that Mario married Margherita Bonareni, a widow from Bergamo, & took over various properties of hers. He lived prosperously in Bergamo with his wife & 2 daughters, Nostra & Arpalice, until his death in 1596.