We report the discovery of a new directly imaged brown-dwarf companion with Keck/NIRC2+pyWFS around a nearby mid-type M-dwarf LSPM J1446+4633 (hereafter J1446). The L' -band contrast (4.5 × 10‑3) is consistent with a ∼20–60 MJup object at 1–10 Gyr and our two-epoch NIRC2 data suggest a ∼30% (∼3.1σ) variability in its L' -band flux. We incorporated Gaia DR3 non-single-star catalog into the orbital fitting by combining the Subaru/IRD radial velocity monitoring results, NIRC2 direct imaging (DI) results, and Gaia proper motion acceleration. As a result, we derive 59.8‑1.4+1.5MJup and ≈4.3 au for the dynamical mass and the semimajor axis of the companion J1446B, respectively. J1446B is one of the intriguing late-T dwarfs showing variability at L' -band for future atmospheric studies with the constrained dynamical mass. Because the J1446 system is accessible with various observation techniques such as astrometry, DI, and high-resolution spectroscopy including radial velocity measurement, it has a potential as a great benchmark system to improve our understanding for cool dwarfs.