Giustini was born in Pistoia, of a family of musicians which can be traced back to the early 17th century; coincidentally he was born in the same year as Bach, Scarlatti, and Handel.
In 1725, on his father's death, Giustini became organist at the Congregazione dello Spirito Santo, and acquired a reputation there as a composer of sacred music: mostly cantatas and oratorios.