The mass and age of substellar objects are degenerate parameters leaving the evolutionary state of brown dwarfs ambiguous without additional information. Theoretical models are normally used to help distinguish between old, massive brown dwarfs and young, low mass brown dwarfs but these models have yet to be properly calibrated. We have carried out an infrared high-contrast imaging program with the goal of detecting substellar objects as companions to nearby stars to help break degeneracies in inferred physical properties such as mass, age, and composition. Rather than using imaging observations alone, our targets are pre-selected based on the existence of dynamical accelerations informed from years of stellar radial velocity (RV) measurements. In this paper, we present the discovery of a rare benchmark brown dwarf orbiting the nearby ($d=18.69pm0.19$ pc), solar-type (G9V) star HD 4747 ([Fe/H]=$-0.22pm0.04$) with a projected separation of only $rho=11.3pm0.2$ AU ($theta approx$ 0.6). Precise Doppler measurements taken over 18 years reveal the companions orbit and allow us to place strong constraints on its mass using dynamics ($m sin(i) = 55.3pm1.9M_J$). Relative photometry ($Delta K_s=9.05pm0.14$, $M_{K_s}=13.00pm0.14$, $K_s - L = 1.34pm0.46$) indicates that HD 4747 B is most-likely a late-type L-dwarf and, if near the L/T transition, an intriguing source for studying cloud physics, variability, and polarization. We estimate a model-dependent mass of $m=72^{+3}_{-13}M_J$ for an age of $3.3^{+2.3}_{-1.9}$ Gyr based on gyrochronology. Combining astrometric measurements with RV data, we calculate the companion dynamical mass ($m=60.2pm3.3M_J$) and orbit ($e=0.740pm0.002$) directly. As a new mass, age, and metallicity benchmark, HD 4747 B will serve as a laboratory for precision astrophysics to test theoretical models that describe the emergent radiation of brown dwarfs.
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