Grassi was born in Savona, then part of the Republic of Genoa. He entered the novitiate of the Society of Jesus at the Church of Sant'Andrea al Quirinale in Rome in 1600. Following his profession of his first religious vows in 1603, he began studies at the Roman College, there taking courses in philosophy, theology and mathematics. Among his professors were the noted German Jesuit mathematicians, Christopher Clavius, Christoph Grienberger and Odo Van Maelcote. He continued his studies there until 1610..
