His great-great-grandfather was Duke Ippolito Lante Montefeltro della Rovere.

He was educated at the Collegio Tolomei in Siena, the Collegio Romano in Rome and the La Sapienza University in Rome where he received a doctorate in utroque iuris (both civil and canon law) in 1824.

Mellini was ordained on 19 September 1818 and was named an Auditor of the Sacred Roman Rota.

He was elected Archbishop of Ravenna and was consecrated in 1826 by Pope Leo XII.

He participated in the Papal Conclave of 1846 which elected Pope Pius IX and was made Secretary of Memorials in 1857.