  We present extensive optical photometric and spectroscopic observations for
the nearby Type Ia supernova (SN Ia) 2019ein, spanning the phases from $\sim 3$
days to $\sim 330$ days after the explosion. This SN Ia is characterized by
extremely fast expansion at early times, with initial velocities of Si II and
Ca II being above ~ 25,000--30,000 km/s. After experiencing an unusually rapid
velocity decay, the ejecta velocity dropped to ~ 13,000 km/s around maximum
light. Photometrically, SN 2019ein has a moderate post-peak decline rate
($\Delta m_{15}(B) = 1.35 \pm 0.01$ mag), while being fainter than normal SNe
Ia by about 40% (with $M^{\rm max}_{B} \approx -18.71 \pm 0.15$ mag). The
nickel mass synthesized in the explosion is estimated to be 0.27--0.31
$M_{\odot}$ from the bolometric light curve. Given such a low nickel mass and a
relatively high photospheric velocity, we propose that SN 2019ein likely had a
sub-Chandrasekhar-mass white dwarf (WD) progenitor, $M_{\rm WD} \lesssim 1.22
M_{\odot}$. In this case, the explosion could have been triggered by a
double-detonation mechanism, for which 1- and 2-dimensional models with WD mass
$M_{\rm WD} \approx 1 M_\odot$ and a helium shell of 0.01 $M_{\odot}$ can
reasonably produce the observed bolometric light curve and spectra. The
predicted asymmetry as a result of double detonation is also favored by the
redshifted Fe II and Ni II lines observed in the nebular-phase spectrum.
Possible diversity in origin of high velocity SNe Ia is also discussed.
