With the search for hydrocarbons moving to more extreme environments, including deepwater, one of the challenges associated with cementing is ensuring the long-term integrity and mechanical properties of the cement at high temperatures (HT). To avoid strength retrogression at temperatures above about 110°C, silica is added to the cement. This makes the hydration process (setting and curing of the cement) more complex, as initially formed hydration products are replaced by more stable phases over time. To study the nanostructure and mechanical properties of HT-cured cement, we applied a variety of techniques including small-angle neutron scattering (SANS), transmission electron microscopy (TEM), nanoindentation, and micro-scratch testing on small cement samples and investigated their structure and properties under a variety of conditions. We observed that, at HT, there is a general coarsening of the nanometer-scale structure of the set cement paste over time, with associated degradation of the properties. We show that the rate of coarsening depends strongly on the initial curing conditions, providing possible strategies for improving the properties and performance of HT-cured cement. The findings may have particular application to geothermal wells and steam injection wells.