Llanocetus was described in 1989 by paleontologist Edward Mitchell based on a partial mandible with two teeth, specimen USNM 183022, and an endocast of the braincase, referred to the same specimen, from the La Meseta Formation of Seymour Island, Antarctica.

[1] The complete skull belonging to the holotype specimen was described in 2018 by paleontologists Robert Ewan Fordyce and Felix Marx.

[5] Phylogenetic analysis grouped Llanocetus into the Eocene family Llanocetidae along with Mystacodon and an as yet undescribed specimen OU GS10897 from New Zealand.

[3] The genus name is in honor of the American biologist George A. Llano who helped Mitchell gain access to the specimen.

The species name derives from Latin denti, teeth, and crena, notch, in reference to the tooth shape which is similar to the crabeater seal.

The dental formula, 3.1.4.23.1.4.3, indicating number of, in order, incisors, canines, premolars, and molars in one half of a jaw, is similar to basilosaurid archaeocetes.

[6] The La Meseta Formation represents the Eocene, which spans 48 to 34 million years ago (mya).

The area probably was a cool-water environment, unlike modern Antarctica, with the still-forming Humboldt Current cycling cold, nutrient-rich water.