jats:titleAbstract</jats:title> jats:pTension remains between the observed and modeled properties of substellar objects, but objects in binary orbits, with known dynamical masses, can provide a way forward. HD 72946 B is a recently imaged brown dwarf companion to a nearby, solar-type star. We achieve ∼100 jats:italicμ</jats:italic>as relative astrometry of HD 72946 B in the jats:italicK</jats:italic> band using VLTI/GRAVITY, unprecedented for a benchmark brown dwarf. We fit an ensemble of measurements of the orbit using jats:monospaceorbitize!</jats:monospace> and derive a strong dynamical mass constraint jats:italicM</jats:italic> jats:sub jats:italicB</jats:italic> </jats:sub> = 69.5 ± 0.5 jats:italicM</jats:italic> jats:subJup</jats:sub> assuming a strong prior on the host star mass jats:italicM</jats:italic> jats:sub jats:italicA</jats:italic> </jats:sub> = 0.97 ± 0.01 jats:italicM</jats:italic> jats:sub⊙</jats:sub> from an updated stellar analysis. We fit the spectrum of the companion to a grid of self-consistent jats:monospaceBT-Settl-CIFIST</jats:monospace> model atmospheres, and perform atmospheric retrievals using jats:monospacepetitRADTRANS</jats:monospace>. A dynamical mass prior only marginally influences the sampled distribution of effective temperature, but has a large influence on the surface gravity and radius, as expected. The dynamical mass alone does not strongly influence retrieved pressure–temperature or cloud parameters within our current retrieval setup. Independently of the cloud prescription and prior assumptions, we find agreement within ±2jats:italicσ</jats:italic> between the C/O of the host (0.52 ± 0.05) and brown dwarf (0.43–0.63), as expected from a molecular cloud collapse formation scenario, but our retrieved metallicities are implausibly high (0.6–0.8) in light of the excellent agreement of the data with the solar-abundance model grid. Future work on our retrieval framework will seek to resolve this tension. Additional study of low surface gravity objects is necessary to assess the influence of a dynamical mass prior on atmospheric analysis.</jats:p>