The fleet of research and training reactors is aging, and no new research reactors are planned in the United States; thus, there is a need to expand the capabilities of existing reactors to meet users' needs. To address these needs, the Colorado School of Mines added a neutron beamline facility to the U.S. Geological Survey TRIGA Reactor (GSTR), a 1-MW(thermal) Mark-I TRIGA reactor located at the Denver Federal Center in Lakewood, Colorado. The original GSTR design did not include any beam ports, and future research efforts will benefit from a neutron beam at the GSTR. Adding new beamline facilities to existing research reactors is both rare and challenging, and this paper describes the design and installation of a new neutron beamline facility at a Mark-I TRIGA reactor with no existing beamline facilities. The design and construction of a radiation beamstop for the new beamline is described in detail. A neutronics model of the neutron beam provides researchers with a useful tool for experiment design. The new neutron beam has a measured length-to-diameter ratio of 200 ± 10, a neutron flux of 2.2×106 ± 6.4×105 n/cm2-s, and an average cadmium ratio of 7.4 using copper, gold, manganese, and indium foils.