  We present the atmospheric characterization of the substellar companion HD
19467 B as part of the pioneering JWST GTO program to obtain moderate
resolution spectra (R$\sim$2,700, 3-5$\mu$m) of a high-contrast companion with
the NIRSpec IFU. HD 19467 B is an old, $\sim$9 Gyr, companion to a Solar-type
star with multiple measured dynamical masses. The spectra show detections of
CO, CO$_2$, CH$_4$, and H$_2$O. We forward model the spectra using Markov Chain
Monte Carlo methods and atmospheric model grids to constrain the effective
temperature and surface gravity. We then use NEWERA-PHOENIX grids to constrain
non-equilibrium chemistry parameterized by $K_{zz}$ and explore molecular
abundance ratios of the detected molecules. We find an effective temperature of
1103 K, with a probable range from 1000--1200 K, a surface gravity of 4.50 dex,
with a range of 4.14--5.00, and deep vertical mixing, log$_{10}$($K_{zz}$), of
5.03, with a range of 5.00--5.44. All molecular mixing ratios are approximately
Solar, leading to a C/O $\sim$0.55, which is expected from a T5.5 brown dwarf.
Finally, we calculate an updated dynamical mass of HD 19467 B using newly
derived NIRCam astrometry which we find to be $71.6^{+5.3}_{-4.6}
M_{\rm{Jup}}$, in agreement with the mass range we derive from evolutionary
models, which we find to be 63-75 $M_{\rm{Jup}}$.These observations demonstrate
the excellent capabilities of the NIRSpec IFU to achieve detailed spectral
characterization of substellar companions at high-contrast close to bright host
stars, in this case at a separation of $\sim$1.6\arcsec with a contrast of
10$^{-4}$ in the 3-5 $\mu$m range.
