Temperature is a key control parameter of biological processes, but measuring and controlling temperatures on a cellular-length scale in living organisms remains an outstanding challenge. Applying nanoscale-thermometry techniques to early embryos, we study cell divisions in a highly controlled manner using local laser heating and real-time in vivo temperature readout. Nitrogen-vacancy centers in nanodiamonds, incorporated into the cells, allow us to map out the temperature distribution of a locally heated embryo with submicrometer spatial resolution and high sensitivity. The simultaneous cell-division imaging under controlled laser heating is used to achieve cell-cycle timing control and inversion, providing insights into timing-regulation mechanisms during early embryogenesis. Understanding the coordination of cell-division timing is one of the outstanding questions in the field of developmental biology. One active control parameter of the cell-cycle duration is temperature, as it can accelerate or decelerate the rate of biochemical reactions. However, controlled experiments at the cellular scale are challenging, due to the limited availability of biocompatible temperature sensors, as well as the lack of practical methods to systematically control local temperatures and cellular dynamics. Here, we demonstrate a method to probe and control the cell-division timing in Caenorhabditis elegans embryos using a combination of local laser heating and nanoscale thermometry. Local infrared laser illumination produces a temperature gradient across the embryo, which is precisely measured by in vivo nanoscale thermometry using quantum defects in nanodiamonds. These techniques enable selective, controlled acceleration of the cell divisions, even enabling an inversion of division order at the two-cell stage. Our data suggest that the cell-cycle timing asynchrony of the early embryonic development in C. elegans is determined independently by individual cells rather than via cell-to-cell communication. Our method can be used to control the development of multicellular organisms and to provide insights into the regulation of cell-division timings as a consequence of local perturbations.