Giulio probably was born in the mid-1490s in Rome, and he joined Raphael's workshop in about 1516. A highly gifted artist, he quickly became Raphael's most trusted assistant, although the two had fundamentally different artistic personalities. On Raphael's death in 1520, Giulio inherited joint control of the workshop, but rapidly moved toward a more expressive and mannered style.
In 1524 Giulio accepted an offer of employment at the Gonzaga court in Mantua, where his primary role was as a designer of projects ranging from palaces to frescoes to spoons. He rarely worked as a painter, and nearly all his plans, including the construction and decoration of the ducal palace and the Palazzo Te, Federico Gonzaga's villa on the outskirts of town, were executed by assistants in accordance with his designs. The attraction of these drawings lies in the fertility of Giulio's imagination and the panache of his pen work. He enjoyed an artistic monopoly in Mantua for two decades until his death in 1546, and the many prints made after his designs ensured his fame throughout Europe.