The launch of app-based for-hire vehicle (FHV) services like Uber and Lyft has led to increased mobility options, but the associated increase in vehicular traffic has also presented challenges. In New York City, the number of FHVs tripled between 2010 and 2019, to over one hundred thousand, due to the advent of such companies. This study seeks to understand the impact this increase in FHV usage has had on greenhouse gas emissions in New York City. The study uses data collected by the NYC Taxi and Limousine Commission, which regulates the FHV and taxi industries, and the NYC Mayor’s Office of Sustainability, which publishes the City’s greenhouse gas emissions inventory. The main result of the study is that although the overall per-vehicle efficiency of the fleet has improved, the high growth in registered vehicles has led to emissions from FHVs and taxis increasing 66 percent from 2010 to 2018. Electric vehicles present an opportunity for emissions reductions in New York City’s FHV fleet if barriers to vehicle adoption are adequately addressed and if adoption of EVs does not outpace vehicle attrition.