Quantifying the global warming potential of existing water infrastructure is an important step in realising the water industry's commitment to net-zero carbon. Whilst there has been an improved understanding of the global warming potential of centralized urban water infrastructure, rigorous analyses of smaller-scale rural systems are rare. This work adopts a life cycle assessment to ascertain the global warming potential of existing drinking water treatment works and wastewater treatment works associated with five Scottish islands: Arran, Iona, Jura, Barra, and Vatersay. The water systems, from source to sink, along with the use of chemicals, transportation, energy, and the disposal of waste products from water infrastructure are considered. The global warming potentials of the island's drinking water treatment works ranged from 0.18 to 0.79 kgCO2-eq/m3 of drinking water, while that for wastewater treatment works were 0.51–1.14 kgCO2-eq/m3 of wastewater. The global warming potential for water services on the islands can be as much as 7 times of that water services across Scotland as previously reported. Major global warming potential contributor in drinking water treatment works was the electricity consumed by the membrane bioreactor. The modelled direct emission of methane from sludge in septic tanks and landfill made the largest contribution to global warming potential. It was also highly sensitive to model parameters, which highlights the need for a comprehensive exploration of process emissions from septic tanks and sludge handling. This analysis of existing rural water infrastructure is a baseline against which potential alternative low-carbon technology configurations can be compared.