A geothermal heat mining system can operate within a single primary borehole in a geothermal reservoir. A primary fluid loop can include a cold working fluid line leading into the primary borehole and a hot working fluid line coming out of the primary borehole. A secondary fluid loop can be located down the primary borehole, where the secondary fluid loop is in thermal contact with the geothermal reservoir. A downhole heat mining device can control a rate of heat transfer from the secondary fluid loop to the primary fluid loop by selectively controlling fluid flow through the primary fluid loop, the secondary fluid loop, or both.