The origins of Valmontone are uncertain: it seems that a village was founded before the rise of Rome on a hill in the modern municipality of the town, and its ruins were visible until the 18th century.

Perhaps these are the remains of the ancient Labicum, which, according to the myth, was founded by Glaucus, Minos’ son: the name of the village derives from a kind of Greek shield.

Labicum was in war against Rome, but at last it was defeated and became a Roman castrum, a fortified castle: other testimonies of the Roman period are the post-station Ad Bivium, situated along the road called Via Latina, a village of coal-makers, some furnaces for tiles and vases, a villa and some other remains (two sarcophagus, memorial plates).

The presence of a Castrum Lateranense goes back to the 1052, while the name of Vallis Montonis (Valmontone means “a valley overhung by a little hill”) appears the first time in a document dated 1139.

On 22 January 1944, during the Italian Campaign of the Second World War, the Allies commenced Operation Shingle, an amphibious landing at Anzio in an attempt to outflank the formidable German defensive positions known as the Winter Line (also Gustave Line) and push toward Rome: Valmontone was an important objective on the way to Rome, in according to Operation Buffalo, May–June 1944.