  We report the discovery of TOI-2180 b, a 2.8 $M_{\rm J}$ giant planet
orbiting a slightly evolved G5 host star. This planet transited only once in
Cycle 2 of the primary Transiting Exoplanet Survey Satellite (TESS) mission.
Citizen scientists identified the 24 hr single-transit event shortly after the
data were released, allowing a Doppler monitoring campaign with the Automated
Planet Finder telescope at Lick Observatory to begin promptly. The radial
velocity observations refined the orbital period of TOI-2180 b to be
260.8$\pm$0.6 days, revealed an orbital eccentricity of 0.368$\pm$0.007, and
discovered long-term acceleration from a more distant massive companion. We
conducted ground-based photometry from 14 sites spread around the globe in an
attempt to detect another transit. Although we did not make a clear transit
detection, the nondetections improved the precision of the orbital period. We
predict that TESS will likely detect another transit of TOI-2180 b in Sector 48
of its extended mission. We use giant planet structure models to retrieve the
bulk heavy-element content of TOI-2180 b. When considered alongside other giant
planets with orbital periods over 100 days, we find tentative evidence that the
correlation between planet mass and metal enrichment relative to stellar is
dependent on orbital properties. Single-transit discoveries like TOI-2180 b
highlight the exciting potential of the TESS mission to find planets with long
orbital periods and low irradiation fluxes despite the selection biases
associated with the transit method.
