Understanding how biodiversity, ecosystem function and species conservation are affected by factors such as climate or land use change, disease and pollution is an important aspect of species conservation and a key focus of the field of Conservation Medicine. Mammals such as squirrels can be valuable biological sentinels for environmental pollution and the intention of this pilot project was to test a new, rapid technique that allows direct detection of mercury (Hg) in hair samples. Our aim was to establish, for the first time, if red squirrels in Europe show any indication of mercury pollution; and to compare levels from red squirrels in two rural UK (Arran and Brownsea Island) and one Polish city population (Warsaw). The latter is exposed to higher levels of Hg from coal-fired power plants. Total mercury levels ranged from 11.1 to 801.95 μg/kg. Contrary to our expectations, females from the Island of Arran had significantly higher Hg values than either males or females from the two other sites. Although the Isle of Arran was the only site where the difference between females and males was significant, our findings for both Poland and the UK suggest that mercury does not only accumulate in marine food chains and the arctic ecosystem, but is present in urban ecosystems and terrestrial woodlands. Data on trends to determine if Hg values are accumulating in arboreal mammals are completely lacking, and point to a need for a monitoring strategy of mercury levels and associated, potential health impacts in endangered species.