The authors agree that this surname derives from the name of Germanic origin Leonardo, documented in the Middle Ages in Latinized forms such as Leonardus, Lionardus, Lonardus and Lunardus. The name is compounded from the elements leo or lewo ("lion", an old Germanic loan from the Latin leo leonis) and hardhu ("hard, strong, valiant"), with the meaning "strong or valiant as a lion". E. Rossoni documents one of its Latinized forms, Lionhardus, in a deed of manumission drawn up in Cremona in May 754.

E. De Felice notes that the surname has a broad and differentiated distribution in Italy: the forms with Lo- predominate in the centre-south; the forms with Lu- are concentrated in Lombardy, Liguria and Veneto; the forms with Le- and Li- are proper to Veneto; and the variant with Lei- predominates in Sardinia. Rossoni broadens this geography with specific regional variants: Lenardi and Lenarduzzi are proper to Friuli; Leonardelli is characteristic of Trentino; Lonardi is concentrated in the Verona area; Lunardi is very widespread in Veneto and northern Tuscany; and Lonardo is clearly Campanian.

The surname Leonardi proper is very widespread throughout Italy, according to Rossoni. De Felice adds that it records a very high frequency in the Catania area.