After high school, Messmer studied computer science at Syracuse University before switching her major to geology.
Messmer has served on then-Governor Maggie Hassan's Task Force to investigate the rhabdomyosarcoma and pleuropulmonary blastoma double pediatric cancer cluster that she identified and reported in 2014.
[6] She was appointed head of the Governor's Task Force Subcommittee to coordinate efforts with the USEPA and New Hampshire Department of Environmental Services (NHDES) to direct the investigation into Coakley Landfill Superfund Site.
House Bill 485, which addressed the need for standards for emerging contaminants in drinking water was opposed by the Business and Industry Association.
[12] On November 15, 2017, Messmer announced her candidacy for the 2018 Democratic nomination for New Hampshire's 1st congressional district, following incumbent representative Carol Shea-Porter's decision not to seek re-election.