He was born in Trino, Piedmont, the son of Theodore II of Montferrat, with whom he collaborated in the government of the marquisate from 1404.
[2] After his father's death in 1418, John Jacob received the investiture as Marquis by emperor Sigismund.
He distinguished himself for some brilliant military campaign in the Apennines area; he also increased his prestige through the marriage between his sister Sofia and the second last Byzantine Emperor, John VIII Palaiologos.
John Jacob's expansion, however, spurred the reaction of the Dukes of Savoy and of Filippo Maria Visconti of Milan.
However, when Amadeus VIII asked him the payment of the war expenses, John Jacob rebelled; but, after having besieged in Chivasso, he was forced to cease.