In previous papers, we presented the discovery of a 12 s X-ray pulsar in the supernova remnant Kes 73, providing the first direct evidence for an ultramagnetized neutron star, a magnetar, with an equivalent dipole field of nearly 20 times the quantum critical magnetic field (m^2_ec^3/eℏ). Our conclusions were based on two epochs of the measurement of the spin, along with an age estimate of the host supernova remnant. Herein, we present a spin chronology of the pulsar using additional Ginga, ASCA, RXTE, and BeppoSAX data sets spanning over a decade. The timing and spectral analyses confirm our initial results and severely limit an accretion origin for the observed flux. Over the 10 yr baseline, the pulsar is found to undergo a rapid, constant spin-down while maintaining a steady flux and an invariant pulse profile. Within the measurement uncertainties, no systematic departures from a linear spin-down are found-departures as in the case of glitches or simply stochastic fluctuations in the pulse times of arrival (e.g., red timing noise). We suggest that this pulsar is akin to the soft gamma-ray repeaters; however, it is remarkably stable and has yet to display similar outbursts. Future gamma-ray activity from this object is likely.