We present a study of strong intervening absorption systems in the near-IR spectra of 31 luminous quasars at z > 5.7. The quasar spectra were obtained with Gemini GNIRS that provide continuous wavelength coverage from similar to 0.9 to similar to 2.5 mu m. We detect 32 strong Mg ii doublet absorbers with rest-frame equivalent width W-r(lambda 2796) > 1.0 A at 2.2 z ii absorber is confirmed by at least two associated Fe ii absorption lines in the rest-frame wavelength range of similar to 1600-2600 A. We find that the comoving line density (dN/dX) of the strong Fe ii-bearing Mg ii absorbers decreases toward higher redshift at z > 3, consistent with previous studies. Compared with strong Mg ii absorbers detected in damped Ly alpha systems at 2 z <4, our absorbers are potentially less saturated and show much larger rest-frame velocity widths. This suggests that the gas traced by our absorbers are potentially affected by galactic superwinds. We analyze the Hubble Space Telescope near-IR images of the quasars and identify possible associated galaxies for our strong absorbers. There are a maximum of two galaxy candidates found within 5 '' radius of each absorber. The median F105W-band magnitude of these galaxy candidates is 24.8 mag, which is fainter than the L* galaxy luminosity at z similar to 4. By using our observed dN/dX of strong Mg ii absorbers and galaxy candidates median luminosity, we suggest that at high redshift, strong Mg ii absorbers tend to have a more disturbed environment but smaller halo size than that at z <1.