Global urbanisation will put considerable stress on both water and energy resources. While there is much research at the national and regional levels on the energy implications of water supply (the urban water-energy 'nexus'), there is relatively little at the city scale. This literature is further diminished when attempting to account for the climate impact of urban water systems. A study of the urban water-energy-climate nexus is presented for México City. It is shown that 50% of México City water comes from a local aquifer with a further 30% deriving from energy-intensive surface sources which are pumped over considerable topography. The water supply system consumes 90% of the water system energy demand, and is responsible for the majority (90%) of the CO2e emissions. In the wastewater sector, 80-90% is discharged with no or little treatment, with correspondingly low energy demand. The small fraction that is treated accounts for the majority of energy use in the wastewater sector. This study shows the uncertainty in energy demand and CO2e emissions when reliant on secondary data which considerably over/under-estimate energy use compared with primary data. This has implications when assessing energy and carbon budgets. Three water savings options are assessed for their impact on energy and CO2e emissions reductions. Considerable reductions in water supply volumes and concomitant energy consumption and CO2e emissions are possible. However the extent of implementation, and the effectiveness of any implemented solutions depend on financing, institutional backing and public support. An additional measure to reduce the climate impact is to switch from traditional to renewable fuels. This work adds city-level quantification of the urban water-energy-climate nexus, allowing policy makers to discern which water-system elements are responsible for the greatest energy use and climate impact, and are better equipped to make targeted operational decisions.