Born to an unknown father, Nannetti was taken in by a charitable institution before being placed, at the age of ten, in a psychiatric hospital for minors. In 1956, he was arrested for insulting a public official and then committed to a psychiatric hospital the same year. Diagnosed with schizophrenia, he was transferred to the psychiatric hospital in Volterra, Tuscany. Taciturn and solitary, he withdrew into silence and claimed to be in contact with electrical and magnetic waves, reporting the news he received telepathically on a daily basis. During the daily walks permitted to patients, Nannetti engraved writings in stone using the metal buckle of his vest, part of the uniform worn by each patient. From 1959 to 1961, and then again from 1968 to 1973, he created a seventy-meter-long fresco that stretched across several walls of the institution's inner courtyard.

In the last years of his life, Nannetti produced nearly 1600 drawings, the vast majority of which were destroyed. Saturated with motifs, his drawings blur the line between writing and drawing, which merge and intertwine like a skillful weaving. Writing loses its signifying function and becomes an abstract form that obeys an iterative graphic rhythm and expresses a tension.