He fought for the Spanish forces in Flanders.
Olivares called him to Madrid, where he arrived in 1636, to become the official chronicler to Philip IV. In 1640 he was one of the ambassadors sent by Philip to England, in an attempt to avert the marriage of Mary Stuart to William II of Orange.
He became adviser to the Cardinal-Infante Ferdinand of Austria from 1643.
Initially he wrote on Tacitus, in the tradition of Justus Lipsius, but as a Christian neo-stoic, and anti-Ciceronian. Olivares, who became Malvezzi's patron, was also a Lipsian. His style imitated Tacitus, too, in its dour compression, and was criticized for its opacity by the translator Thomas Powell; another view is that his prose was "elegantly laconic". John Milton referred to "Malvezzi, that can cut Tacitus into slivers and steaks".
His political thought was in the tradition of Machiavelli. His Tarquin argues the case for dissimulation in politics.
His biography of Olivares (Ritratto del Privata Politico Christiano) has been called hagiography. It argued that he was right to invoke the reason of state on behalf of the Spanish Empire.
He wrote in Italian and Spanish, and was early translated into Latin, Spanish, German and English, with a Dutch edition of 1679.