The scholars consulted agree that this surname derives from an ethnonym or demonym designating someone from the region of Lombardy or belonging to the Lombard people — the Germanic tribe that settled in Italy from the 6th century onward. Over time, "lombardo" came to designate the inhabitants of that region more generally, and the surname likely arose as a designation for those who had migrated from northern Italy to other areas and were identified by their provenance.

F. Miranda adds that the surname may also originate in various place names, such as Alzano Lombardo (in the area of Bergamo), Martirano Lombardo (in the area of Catanzaro), or San Vito dei Lombardi (in the area of Avellino), or from the transformation into a surname of the medieval Italo-Germanic personal name Lombardo. Miranda further notes that the Latin etymon langobardus or longobardus is compounded from lang ("long") and bart ("beard"), making longobardi the equivalent of "men of the long beard," according to the chronicler Paul the Deacon.

P. Lastrico suggests that the surname could have originated not only from geographic provenance but also as an ironic or evaluative nickname, since in certain contexts being a "Lombard" carried stereotyped characteristics — including being hardworking, cold, or calculating — or alternatively as an honorific nickname tied to the reputation of Lombards as skilled bankers and craftsmen.

E. Rossoni notes that the form Lombardo is more characteristic of the south, particularly Sicily and Calabria, while Lombardini is more prevalent in Lombardy and other central-northern regions such as Emilia-Romagna, Tuscany, and Lazio. Rossoni also identifies Lombardelli as found in central-northern Italy — specifically in the areas of Piacenza, Ferrara, Tuscany, and Marche — and flags Lombardello as possibly a misrecording of that variant. Regarding Lombardia, Rossoni considers it a rare form possibly derived from a regional nickname or attributed to a foundling. As for Longobardi, it is described as very common in Campania and Rome, while Longobardo, rarer and found in Naples, derives from the medieval use of "Longobardia" to refer to the broader area encompassing Campania and Apulia — meaning it may have denoted a resident of those regions rather than a descendant of the Germanic tribe.

The surname is characterized as properly Siculo-Calabrian in its strongest concentration, yet widespread across all of Italy, with the greatest density in Sicily and in southern Italy, especially Calabria, Campania, and Apulia, and with significant presence also in Lombardy, Piedmont, Lazio, Liguria, Veneto, Tuscany, and Emilia-Romagna. Documented variants include Lombardi, Lombardelli, Lombardino, Lombardini, Lombardozzi, Longobardi, and Longobardo.