Early embryos display a remarkable ability to regulate the patterning of tissues in response to changes in tissue size. However, it is not clear whether this ability continues into post-gastrulation stages upon cell commitment to distinct germ layers. Here, we performed targeted removal of neural fated cells in the zebrafish tailbud using multi-photon ablation. This led to a proportional reduction in the length of both the spinal cord and paraxial mesoderm in the tail, revealing a capacity to regulate tissue morphogenesis across multiple tissues to build a well-proportioned posterior body. Following analysis of cell proliferation, gene expression, signalling and cell movements, we found no evidence of cell fate switching from mesoderm to neural fate to compensate for neural progenitor loss. Furthermore, we found that tail paraxial mesoderm length is not reduced upon direct removal of an equivalent number of mesoderm progenitors, ruling out the hypothesis that neuromesodermal competent cells enable proportional regulation. Instead, reduction in the numbers of cells across the spinal cord reduces both spinal cord and paraxial mesoderm length. We conclude that spinal cord elongation is a driver of paraxial mesoderm elongation in the zebrafish posterior body and that this can explain proportional regulation of both tissues upon neural progenitor reduction.