Reactor antineutrino detection has matured to the point where it is feasible to stably monitor plutonium content and reactor power using a high statistics (hundreds or thousands of counts per day) antineutrino detector at a standoff of a few tens of meters. Our Lawrence Livermore National Laboratory/Sandia National Laboratories collaboration has deployed a detector to demonstrate this capability at a 3 GW pressurized water reactor in Southern California, operating 25 meters from the core center, and acquiring data over an approximate one year period. Such monitoring may be useful for non-intrusively tracking power output and plutonium buildup in nuclear reactors, providing the earliest possible measurement of the amount of plutonium in the reactor core. We present our antineutrino event sample, and show that change in antineutrino rate recorded in our detector over time is consistent with the expected systematic decline in rate induced by growth of plutonium as the reactor proceeds through its cycle. We estimate the attainable precision of the method, and discuss the benefits of this technology.