We report the beginning of activity for comet C/2015 ER61 (PANSTARRS), the first instance of watching a long-period comet turn on. Pre-discovery observations and observations from the NEOWISE space telescope suggest that the nucleus is large, with a radius of R N  ∼ 9 km, assuming an albedo of 0.025. Our photometric data follows the comet from r = 8.9 to 4.8 au as it moved into solar conjunction in 2016 July. Our sublimation model shows that activity began near r = 8.8 au (true anomaly, TA = −139°) in early 2015, driven by CO2 sublimation, which peaked in 2016 April at r = 5.1 au (TA = −127°). Appreciable water sublimation began around r = 5.0 au. Our sublimation model is consistent with an active water sublimation area of 1% of the surface (equivalent to 10.2 km2), and an active surface area for CO2 sublimation of 0.029% (0.3 km2). The CO2 production rate at r = 4.66 au as measured by NEOWISE is (8.4 ± 2) × 1025 s−1. If CO2-ice had been present on the surface, dust dragged from the surface by sublimation would have been observed much farther out—as far as 20 au. Our thermal models suggest that the CO2 ice was present at a depth of 0.4 m. The comet came out of solar conjunction in 2016 December and, unless it brightens significantly, is unlikely to have water production rates much higher than a few ×1028 s−1.