  We present the joint astrometric and direct imaging discovery, mass
measurement, and orbital analysis of HD 63754 B (HIP 38216 B), a companion near
the stellar-substellar boundary orbiting ~20 AU from its Sun-like host. HD
63754 was observed in our ongoing high-contrast imaging survey targeting stars
with significant proper-motion accelerations between Hipparcos and Gaia
consistent with wide-separation substellar companions. We utilized archival
HIRES and HARPS radial velocity (RV) data, together with the host star's
astrometric acceleration extracted from the Hipparcos-Gaia Catalog of
Accelerations (HGCA), to predict the location of the candidate companion around
HD 63754 A. We subsequently imaged HD 63754 B at its predicted location using
the Near Infrared Camera 2 (NIRC2) in the $L'$ band at the W. M. Keck
Observatory. We then jointly modeled the orbit of HD 63754 B with RVs,
Hipparcos-Gaia accelerations, and our new relative astrometry, measuring a
dynamical mass of ${81.9}_{-5.8}^{+6.4} M_{jup}$, an eccentricity of
${0.260}_{-0.059}^{+0.065}$, and a nearly face-on inclination of
$174.81_{-0.50}^{+0.48}$ degrees. For HD 63754 B, we obtain an L' band absolute
magnitude of $L' = 11.39\pm0.06$ mag, from which we infer a bolometric
luminosity of $log(L_{bol}/L_{\odot})= -4.55 \pm0.08$ dex using a comparison
sample of L and T dwarfs with measured luminosities. Although uncertainties
linger in age and dynamical mass estimates, our analysis points toward HD 63754
B's identity as a brown dwarf on the L/T transition rather than a low-mass
star, indicated by its inferred bolometric luminosity and model-estimated
effective temperature. Future RV, spectroscopic, and astrometric data such as
those from JWST and Gaia DR4 will clarify HD 63754 B's mass, and enable
spectral typing and atmospheric characterization.
