Cosimo's patronage of the visual arts was no less impressive than his support of humanist scholars and literature. Under his bullish, benevolent reign, Filippo Brunelleschi created the vast dome of the cathedral, an architectural miracle, and the sculptor Lorenzo Ghiberti realized the sumptuous gilt bronze Gates of Paradise for the Baptistery portal nearby. While Brunelleschi built for Cosimo what would become the principal Medici church, San Lorenzo, the dependable architect Michelozzo di Bartolomeo, at Cosimo's request, renovated the church and convent of San Marco (where Cosimo maintained a penitential cell), constructed a grand new palace for the Medici on the via Larga, and renovated villas for them in the sylvan backwaters of Cafaggiolo, Careggi, and Trebbio. From his favorite artist, the sculptor Donatello, Cosimo commissioned several important works, including the controversial, sensuous bronze David, a rakish interpretation of the biblical hero, with more sashay than swagger.

Cosimo kept busy the rambunctious Fra Filippo Lippi, seducer of ladies and nuns alike, and all of the other major painters as well; Lippi supplied a number of pictures for the new Palazzo Medici, and Fra Angelico and Paolo Uccello painted altarpieces and frescoes for the churches of San Marco and Santa Maria Novella. Some of Lippi's pictures, such as his Annunciation (c. 1439) in San Lorenzo, followed what were the latest, progressive, “scientific” trends in art. He defined the space of the painting through a strict, if vertiginous, one-point perspective system – a striking application of mathematics and geometry – and he rendered the foreground glass vase with impressive, optical precision.