  We report on the hard X-ray burst and the first ~100 days NICER monitoring of
the soft X-ray temporal and spectral evolution of the newly-discovered magnetar
Swift J1818.0-1607. The burst properties are typical of magnetars with a
duration of $T_{90}=10\pm4$ ms and a temperature of $kT=8.4\pm0.7$ keV. The
2--8 keV pulse shows a broad, single peak profile with a pulse fraction
increasing with time from 30% to 43%. The NICER observations reveal strong
timing noise with $\dot{\nu}$ varying erratically by a factor of 10, with an
average long-term spin-down rate of
$\dot{\nu}=(-2.48\pm0.03)\times10^{-11}$~s$^{-2}$, implying an equatorial
surface magnetic field of $2.5\times10^{14}$ G and a young characteristic age
of $\sim$470 yr. We detect a large spin-up glitch at MJD 58928.56 followed by a
candidate spin-down glitch at MJD 58934.81, with no accompanying flux
enhancements. The persistent soft X-ray spectrum of Swift~J1818.0-1607 can be
modeled as an absorbed blackbody with a temperature of ~1 keV. Its flux decayed
by ~60% while the modeled emitting area decreased by ~30% over the NICER
observing campaign. This decrease, coupled with the increase in the pulse
fraction points to a shrinking hot spot on the neutron star surface. Assuming a
distance of 6.5 kpc, we measure a peak X-ray luminosity of $1.9\times10^{35}$
erg/s, lower than its spin-down luminosity of $7.2\times10^{35}$ erg/s. Its
quiescent thermal luminosity is $\lesssim 1.7\times10^{34}$ erg/s, lower than
those of canonical young magnetars. We conclude that Swift J1818.0-1607 is an
important link between regular magnetars and high magnetic field rotation
powered pulsars.
