Green herons are carnivorous, mainly eating fish and invertebrates. They eat almost anything that they can find and capture. Their invertebrate diet includes: Hirudinea, Oligochaeta, Anisoptera, Anisoptera, Coleoptera, Orthoptera, and Malacostraca. Some of the many fish eaten are: Cyprinidae, Centrarchidae, Ictaluridae, Percidae, eels, and, in urban areas, Carassius auratus. Other vertebrates eaten are Rodentia, Squamata, Anura, Amphibia, and Squamata.

The heavy bill of a green heron enables them to capture large prey. Feeding can take place at any time, day or night. Typically, prey is captured with a darting stroke of the head and neck, lunging the body towards the victim and either grabbing or impaling the prey. The most common feeding technique is to stand in a crouched position with neck and head retracted. Standing often alternates with slow walking in a crouched posture in the water or bordering vegetation. Herons use their feet to stir up animals in the water, making them move and then capturing them. They may also dive from perches head first into deep water, although this isn't a very efficient method. Green herons do best catching prey in shallow water (up to 10 cm deep).

Green herons are one of the few tool-using birds. They use a variety of baits and lures, such as crusts of bread, mayflies, and feathers. They then put the bait on the water surface and wait for prey to attack the bait. They stand motionless near the bait until a small fish or other animal approaches and then grab them.

Animal Foods: mammals; amphibians; reptiles; fish; insects; terrestrial non-insect arthropods; terrestrial worms; aquatic or marine worms