This data presents the first city-wide direct street-level methane (CH4) measurements and emission quantification of Greater London natural gas utilities. Extensive vehicle surveys were conducted between 2018 and 2019 measuring street-level mixing ratios of CH4 and ethane (C2H6) across Greater London. Leak indicators and emission rates were detected and calculated with an algorithm previously developed for mobile surveys. Air samples were collected within elevated plumes of CH4 and δ13CCH4 source signatures determined by stable isotopic composition. Every vehicle-accessible kilometre square in Greater London was covered, which included ~2930 km of the street network, primarily on local arterial and collector roads. Three leak indications per kilometre and an overall emission factor of 0.42 L min-1 km-1 were determined. δ13C signatures and C2:C1 ratios indicated natural gas sources contributed to ~90% of the emissions. Total estimated CH4 emission range (1900 to 2542 tonnes CH4 Yr-1) was comparable to the annual domestic gas consumption of roughly 3000 homes. Data demonstrates the need for targeted mitigation of the larger emissions as ~5% of locations are responsible for ~50% of the total emissions. Prioritization of repair using comprehensive emissions mapping can be a useful greenhouse gas mitigation strategy as the UK seeks to achieve its “Net Zero” emission target.