Leo was born in San Vito degli Schiavoni (current San Vito dei Normanni, province of Brindisi), then part of the Kingdom of Naples.
He became a student at the Conservatorio della Pietà dei Turchini at Naples in 1703, and was a pupil first of Francesco Provenzale and later of Nicola Fago. It has been supposed that he was a pupil of Pitoni and Alessandro Scarlatti, but he could not possibly have studied with either of these composers, although he was undoubtedly influenced by their compositions. His earliest known work was a sacred drama, L'infedelta abbattuta, performed by his fellow-students in 1712.
Giuditta is based on the Biblical story of Judith, a beautiful Israelite widow who insinuates herself within the camp of the conquering Assyrian tribe and deceives their general, Holofernes, before decapitating him and carrying off his head in triumph. This is the second version of the oratorio, known as the Cambridge version after the location of the manuscript when it was turned up in modern times. Scarlatti had considered the original version, composed for Rome in 1693, to be his finest oratorio. This is no mean assessment from the composer of masterworks which have been more celebrated in our own time such as La Maddalena, for their expressive pathos and superbly grateful vocal lines.
Alessandro Scarlatti is a great man but his compositions are very difficult, in a theatre audience of a thousand people only 20 will understand them, thus said Count Francesco Zambeccari, an influential contemporary, and it is a testimony of the skill, complexity and depth of his rich music, a far cry from the facile and fashionable composers of his day. The Oratorio per la Santissima Trinita was composed in 1715, written at the mature age of 50, specifically intended for performance in Naples. The music is at the service of the drama, in a musical action that flows almost without caesura, presenting the richness of Scarlattis invention, always backed up by extremely in-depth knowledge of all the best composition techniques of the long tradition of the Italian School.