Although the exact date he began working in his father's workshop is unknown, experts have found traces of his workmanship beginning about 1670 in instruments labeled as Andrea Guarneri.
His early work shows not only promising ability and craftsmanship, but also a pronounced tendency to express his own personality and follow his own ideas.
He quickly developed his own style, noticeable in its distinctive f-holes, precise corners and purfling, and a fuller arching than his father used.
In 1694, he appears in the census in Mantua as having remarried to Lucia Guidi, and having five children, none of whom followed their father into violin making.
By 1685, he was accomplished enough to perform for the Duke of Mantua as a violin soloist, and play a viol in the Mantuan Court Orchestra.
In general Pietro's instruments were the standard 14 inches long, and tended to be of a narrow pattern with rather elongated bouts.
His massive scrolls are especially distinctive in this period, with the intermediate turn of the volute being left so high that the eyes appear dwarfed.