He trained with the architect-engineer Domenico Fontana and collaborated as a junior partner with Giacomo Della Porta,[1] whom he succeeded as the papacy's chief architect, as "officium architecti nobilis fabricae Capitolii Populi Romani".

In his official capacity Rainaldi also designed the palazzo to house the Jesuits in the Piazza del Gesù, a Mannerist façade without a trace of Baroque in its details.

He also designed the Tombs in Santa Cecilia in Trastevere and collaborated in the baroque setting of the gardens of Villa Borghese.

In the Farnese stronghold of Caprarola, near Rome, Cardinal Odoardo Farnese commissioned Rainaldi to build for the Discalced Carmelites the Church of SS Maria e Silvestro (1621–23), a beautiful and original church adapted to exigencies of the site.

For Francesco d'Este, who, with the loss of the Este seat of Ferrara to the Papal States, concentrated his patronage in his Duchy of Modena, Rainaldi contributed to the construction of the Ducal Palace to supplant the ancient castello, and was in particular charged with the layout and elaborate hydraulics of its gardens, with giochi di acque and a theater clipped in green hedges, 1631-34 (Roganti).