The extinct marsupial 'lion' Thylacoleo carnifex was Australia's largest mammalian carnivore. Despite being the topic of more discussion than any other extinct Australian marsupial (excepting perhaps the Thylacine), basic aspects of its palaeobiology, including its locomotory repertoire, remain poorly understood. Recent discoveries allowed the first reconstruction of an entire skeleton including the first complete tail and hitherto-unrecognised clavicles. Here we describe these elements and re-assess the biomechanics of the postcranial skeleton via comparisons with a range of extant terrestrial, scansorial and arboreal Australian marsupials. Our analysis suggests that T. carnifex possessed: a relatively stiff tail comprising half of the vertebral column length; proximal caudal centra exhibiting a relatively high resistance to sagittal and lateral bending (RSB and RTB); relatively enlarged areas of origin for caudal flexors and extensors; a rigid lumbar spine; and a shoulder girdle braced by strong clavicles. The lever arms of major muscle/tendon systems controlling the axial and appendicular skeleton were identified and RSB and RTB calculated. The combination of these features compared most closely overall with those of the much smaller Tasmanian Devil (Sarcophilus harrisii), a hunter/scavenger capable of climbing. Similar locomotor behaviour is proposed for Thylacoleo carnifex. Orientation of articular facets and RSB stresses also indicate that T. carnifex may have held its tail in a dorsally-flexed position.