Problem: To provide a thermally conductive sheet precursor exhibiting excellent thermal conductivity and dielectric breakdown resistance, a thermally conductive sheet obtained from the precursor, and a production method thereof. Solution: The thermally conductive sheet precursor according to an embodiment of the present disclosure includes isotropic thermally conductive aggregates in which anisotropic thermally conductive primary particles are aggregated, an anisotropic thermally conductive material not constituted by the aggregates, and a binder resin; wherein upon the application of a pressure from 3 to 12 MPa to the thermally conductive sheet precursor, at least some of the isotropic thermally conductive aggregates collapse.