Comprehensive National Mercury Monitoring Act - Directs the Administrator of the Environmental Protection Agency (EPA) to establish a national mercury monitoring program that monitors: (1) long-term changes in mercury levels and important ancillary measures in the air; and (2) long-term changes in mercury and methyl mercury levels and important ancillary measures in water and soil or sediments and in marine, freshwater, and terrestrial organisms.

Requires the program to be compatible with similar international efforts, including the Global Mercury Observation System and the Global Earth Observation System of Systems.

Requires the Administrator to: (1) select multiple monitoring sites representing multiple ecoregions that include national parks, wildlife refuges, National Estuarine Research Reserve units, and other sensitive ecological areas in which substantive changes are expected from reductions in domestic mercury emissions; (2) establish and publish standardized measurement protocols for the program; and (3) establish a centralized database for environmental mercury data.

Requires the Administrator to: (1) establish a Mercury Monitoring Advisory Committee to advise the Administrator on the establishment, site selection, measurement, recording protocols, and operation of the program; (2) report on the program every two years and include, every four years, an assessment of mercury deposition rates that need to be achieved in order to prevent adverse human and ecological effects; and (3) make all data obtained under this Act available to the public through a dedicated website and on written request.