Gamma-ray burst ( GRB ) 150910A was detected by Swift /BAT , and then rapidly observed by Swift /XRT , 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 _ { R, 2 } \approx - 1.36 ) , as predicted in the standard external shock model , while the X-ray emission exhibits a plateau ( \alpha _ { X, 1 } \approx - 0.36 ) followed by a steep decay ( \alpha _ { 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 } ~ { } { G } \leq B _ { p } \leq 1.80 \times 10 ^ { 15 } ~ { } { G } and 1 ms \leq P _ { 0 } v \leq 1.77 ms , and the radiative efficiency of the wind is \eta _ { w } \geq 32 \% .