Leonello was one of three illegitimate sons of Niccolò d'Este III and Stella de' Tolomei. He received a military education under the "condottiero" Braccio da Montone, and was tutored by Guarino Veronese, later appointed professor at the University of Ferrara, who also instructed him on the traits of a desirable ruler and how to govern. In 1425, after the execution of his elder brother Ugo Aldobrandino, he was the sole heir of Niccolò. In 1435, he married Margherita Gonzaga on 6 February and was recognized as legitimate son by Pope Martin V. Margherita, who died in 1439, gave birth to a child, Niccolò, in 1438 (died in 1476). In late December 1441, he succeeded to his father's possessions after Niccolò's death in northern Italy.