The giraffe neck is an icon of evolutionary biology. While giraffes maintain the usual mammalian cervical number of seven vertebrae, their 1st thoracic vertebra (T1) exhibits aberrant anatomy and has been hypothesized already more than a century ago to functionally elongate the neck. We quantitatively test this ‘functional elongation hypothesis’ by combining phylogenetically informed analyses of neck length, three-dimensional (3D) vertebral shape, and of the functional significance of shape differences across a broad sample of cetartiodactyls. 3D bone models of the cervical series and cervicothoracic transition were analysed. 3D geometric morphometric analysis revealed that the shape of the 7th cervical (C7) has converged in several long-necked species, however, we find unique homeotic shifts that result in a ‘cervicalization’ of the giraffe’s T1. For the European bison, we demonstrate a ‘thoracalization’ of C7. Other giraffids (okapi and extinct Sivatherium) did not exhibit such ‘cervicalized’ T1 morphology. Quantitative range of motion (ROM) analysis at the cervicothoracic transition in our sample confirms the ‘functional elongation hypothesis’ for the giraffe in terms of increased mobility, especially in regard of dorso-ventral flexion/extension. Other factors related to the unique morphology of the giraffe’s cervicothoracic transition such as neck posture and intervertebral stability may have also contributed to giraffe neck evolution.