The scholars consulted agree that this surname derives from the Latin term molinarius, which designated the owner, manager, or worker of a mill. The occupational name circulated as a byname during the Middle Ages and gradually became a hereditary surname over the course of that period. It is therefore a surname of occupational origin, pointing not to noble titles but to a trade essential to Italian rural life, frequently handed down from generation to generation.

E. De Felice places the transformation into a surname from the already widespread use of molinaro and its regional variants in the late Middle Ages, between the 12th and 13th centuries. Among those variants he mentions mulinaro, the Tuscan-Umbrian mugnàio — probably influenced by the French meunier — and the forms monaro, munaro, and munèr, characteristic of the Veneto but also present in Emilia-Romagna and other areas of northern Italy.

F. Miranda stresses that the surname is present throughout Italy, with a particular concentration in Lombardy, Emilia-Romagna, Liguria, Lazio, Piedmont, Veneto, and Trentino-Alto Adige. In Sicily its presence is more limited, found mainly in the areas of Agrigento, Trapani, Palermo, Catania, and Enna. Miranda also notes the existence of variants arising from dialect alterations of the underlying Latin term.

It is further recorded that the northeastern Italian forms Munari, Munèr, Munerati, and Munarini, as well as the Tuscan and Umbrian Mugnai, Mugnani, and Mugnaini, are all regional adaptations of the same occupational word and share the same origin as Molinari.