This genus lived approximately 115 million years ago during the Early Cretaceous period (Aptian-Albian stages), in shallow seas covering parts of what is now Australia.

All known specimens come from the Bulldog Shale in South Australia, although material very similar to Umoonasaurus has been found in the Darwin Formation in the Northern Territory.

[2] The generic name is a combination of the Antakirinja name for the Coober Pedy region, Umoona, and the Greek word sauros, meaning "lizard."

The specific name comes from the Greek words demos and scylla, meaning "of the people" and "sea monster," respectively, referring to the public donations used to acquire the holotype.

The vomer (a palatal bone close to the tip of the snout) barely extends posteriorly past the internal nares (opening for the nostrils on the inside of the skull).

The basicranium (braincase floor) has gracile paroccipital processes (extensions that connect to other bones in the skull).

[4] Prior to their description, the specimens that would later be assigned to Umoonasaurus were thought to represent a new species of Leptocleidus.

[5] In 2006, Kear and colleagues found Umoonasaurus to belong to the superfamily Pliosauroidea and be the most basal member of the family Rhomaleosauridae.

[9] A 2013 study by Benson and colleagues found Umoonasaurus to be a close relative of Leptocleidus and to belong to Leptocleidia within Plesiosauroidea.

[10] In 2015, Parrilla-Bel and Canudo found Umoonasaurus to be a leptocleidid, and in turn finding Leptocleididae to belong to Leptocleidia, which was once again recovered as a member of Plesiosauroidea.

Topology of Leptocleidia recovered by Parrilla-Bel and Canudo (2015)[11] Plesiopleurodon Edgarosaurus Leptocleidus Nichollssaura Umoonasaurus Vectocleidus Hastanectes Brancasaurus

[1] Preserved gut contents of the type specimen include 60 gastroliths and 17 isolated vertebrae of an indeterminate teleost fish.

[15][3] This indicates a preference for small prey items, a view reinforced by the lack of hypercarnivorous adaptations in Umoonasaurus.

[1] All known Umoonasaurus come from the Bulldog Shale, a member of the Marree Subgroup located in the Eromanga Basin of South Australia.

The presence of glendonite, ice-rafted boulders, and coniferous driftwood with dense growth rings indicate a seasonal climate with near-freezing temperatures.