Air quality studies often take advantage of the ability of lichens and mosses to accumulate certain contaminants in their tissues. In order to evaluate the rate of contaminant accumulation from concentrations in tissues, the age of the tissue must be known. Our objective was to develop and test a method for estimating annual growth rates and thallus ages for Masonhalea richardsonii, a vagrant clonal lichen abundant throughout its Beringean range. The annual growth rate of healthy individuals from Arctic Alaska, based on the 75th percentile of a cumulative frequency distribution, was 11.6% (90% confidence interval: 11.2–13.3%). This growth rate suggests that the biomass of a healthy M. richardsonii individual doubles within a period of approximately 6.3 years (90% confidence interval: 5.6–6.5 years). Average ages for the thalli in this study were estimated at 5 to 41 years using a negative compound interest formula and assuming initial masses of thallus fragments. Calculations and their appropriate application are shown and the methodology is discussed.