Protohadros had a massive, very deep set of lower jaws, and the snout was strongly turned down at the front, which according to Head suggested a habit of grazing on low-growing plants, rather than browsing from bushes or overhanging branches. Its diet would then have consisted of the swamp plants which grew in the delta streams in its habitat, scooped up by the broad, down-turned mouth.
Protohadros had an unusually extensive lower jaw system, with a snout that was significantly angled downwards at the front, as proposed by Head. This likely signified a diet centered on low-growing plant life, rather than browsing higher branches or shrubs. The animal's diet would have consisted of swamp plants found in the delta streams of its habitat, gathered using its broad and downward-facing mouth.