  Gamma-ray burst (GRB) 150910A was detected by {\it Swift}/BAT, and then
rapidly observed by {\it Swift}/XRT, {\it Swift}/UVOT, and ground-based
telescopes. We report Lick Observatory spectroscopic and photometric
observations of GRB~150910A, and we investigate the physical origins of both
the optical and X-ray afterglows, incorporating data obtained with BAT and XRT.
The light curves show that the jet emission episode lasts $\sim 360$~s with a
sharp pulse from BAT to XRT (Episode I). In Episode II, the optical emission
has a smooth onset bump followed by a normal decay ($\alpha_{\rm R,2} \approx
-1.36$), as predicted in the standard external shock model, while the X-ray
emission exhibits a plateau ($\alpha_{\rm X,1} \approx -0.36$) followed by a
steep decay ($\alpha_{\rm X,2} \approx -2.12$). The light curves show obvious
chromatic behavior with an excess in the X-ray flux. Our results suggest that
GRB 150910A is an unusual GRB driven by a newly-born magnetar with its
extremely energetic magnetic dipole (MD) wind in Episode II, which
overwhelmingly dominates the observed early X-ray plateau. The radiative
efficiency of the jet prompt emission is $\eta_{\gamma} \approx 11\%$. The MD
wind emission was detected in both the BAT and XRT bands, making it the
brightest among the current sample of MD winds seen by XRT. We infer the
initial spin period ($P_0$) and the surface polar cap magnetic field strength
($B_p$) of the magnetar as $1.02 \times 10^{15}~{\rm G} \leq B_{p} \leq 1.80
\times 10^{15}~{\rm G}$ and 1~ms $\leq P_{0}v\leq 1.77$~ms, and the radiative
efficiency of the wind is $\eta_w \geq 32\%$.
