Excess extragalactic contribution to residual rotation measure (RRM)for quasars for sightlines with intervening Mg II absorbers is a powerful tool to investigate the presence of magneto-active plasma and to estimate the strength of the magnetic field in high-redshift
galaxies. To carry out such analysis, we have compiled a large sample of 1135 quasars for which we have RRM data as well as optical spectra capable of ascertaining the presence/absence of the intervening MgII
absorbers. In this study, we found that the dispersion in RRM (sigma_rrm) for 315 sightlines having MgII intervening systems is 15.92+/-0.79 rad/m^2 as compared to its value of 14.95+/-0.53 rad/m^2 for the 820 sightlines without such absorbers. This result in an excess standard deviation (sigma_rrm^ex) of 5.46+/-2.72 rad/m^2 among these two subsample. It is also noticed that there is an increasing trend in sigma_rrm for larger rest frame equivalent width (EW_r), having values 13.08+/- 1.30 rad/m^-2 and 15.24+/-1.52
rad/m^2 for EW_r 1~ and EW_r1,respectively. Additionally, we found a strong anti-correlation between sigma_rrm and fractional polarisation (p) with the Pearson correlation coefficient (rho_p) of -0.80 for sightlines with MgII absorbers which is absent for sightlines without any Mg II absorber. This gives support to the hypothesis that intervening Mg II
absorbers do have magnetic field of sufficient strength, and perhaps oriented in random directions, leading to a scatter in RRM and hence
also causing a reduction in the resultant fractional polarization. All these observational evidences, allow us to infer the presence of magnetised plasma in intervening high redshift galaxies with strengths
similar to that in nearby galaxies.