Whether powered by a battery or energy harvested from the environment, low-power (LP) sensor devices require extreme energy efficiency. These sorts of devices are becoming pervasive, running increasingly sophisticated applications in inhospitable environments. We present MINERGY, an energy-efficient microcontroller (MCU) augmented with a vector-dataflow (VDF) co-processor. The testchip taped out on a 22nm bulk finFET CMOS process demonstrates that MINERGY is 60% more energy-efficient than a baseline, scalar, low-power MCU, achieving peak efficiency of 256 MOPS/mW (2.6× prior work) while consuming only 19.1µW (@4MHz). To make the system viable for intermittently powered applications that require nonvolatile storage, MINERGY includes a 256KB embedded MRAM.