The consulted works concur in tracing this surname to the Latin given name Marius. A. Bongioanni identifies it as the name of the gens Maria, whose members claimed descent from the god Mars, with whom it shares its etymology. He notes that Saint Marius, a martyr of the 1st century, is celebrated on January 19, and cautions that the name is sometimes mistakenly read as the masculine form of Maria.

E. Rossoni specifies that the surname and its variants derive, directly or through augmentative forms, from the Latin name Marius, recalling the celebrated Roman general who was the protagonist of the civil war against Sulla. Mario shows areas of presence in the Brescia area, in the Padua and Belluno areas, and in the belt encompassing the Rome, Latina, Caserta, and Naples areas, with a lesser presence in the Bari area.

Rossoni further documents several variants. Marion is concentrated in Veneto — especially in the Treviso and Venice areas — and in Trieste. Marione is very infrequent and appears in northern Italy, likely in Veneto and in the Ligurian-Piedmontese area. Marioni shows a cluster between the Pavia and Milan areas, another in the Udine area, and smaller presences in the La Spezia area, in the Perugia area, and in Tuscany.

It is also recorded that the given name Mario may ultimately derive from the Etruscan word maru, meaning "man."