The Leonini Horti dates back to the period of reconstruction immediately after the long war that culminated with the fall of the Republic of Siena in 1558, commissioned and built by Diomede Leoni in conjunction with the restoration of the ancient walls. Monsignor Lattanzi, on November 8, 1581, wrote that "within the said Land of San Quirico, already very ruined by war, Diomede Leoni was the first who began to restore the ruins and make some of his Gardens called Leonini, which return to embellish that place where he was born and to give comfort to travelers and especially nobles ...". The realization of this work comes from the conception of the garden for public use, and for the wayfarers.

The garden occupies an area of 13,824 square meters and is structured on two levels. The lower level is formed by boxwood regains (Buxus sempervirens), while the upper level is formed by holm oaks (Quercus ilex), according to a central load-bearing axis formed by a long staircase that leads from the entrance to the upper area, with a table with two-faced Janus in the center to mark the passage between the two areas.

At the centre of the parterre and boxwood flowerbeds is a statue of Cosimo III dei Medici made in 1688 by the Sienese sculptor Giuseppe Mazzuoli on commission of Cardinal Flavio Chigi, and numerous inscriptions in Latin written by Leoni.

From the entrance of the garden and along the left side runs a row of oaks, at the bottom of which is the Garden of Roses, which occupies a small space overlooking the Parish of Santa Maria, near the end of the Leonini Horti.

A study conducted by the Department of Biology of Plants of the University of Florence has surveyed 313 trees and found 156 hedges; many are the plants of sloggin (Viburnum tinus) and cypress (Cupressus spp.).

Every year the Leonini Horti host the exhibition of sculpture Forme nel Verde (Forms in Green), an event that proposes the theme of contemporary interventions in historic greenery.