  We present the catalogue of the Mg II absorption systems detected at a high
significance level using an automated search algorithm in the spectra of
quasars from the twelfth data release of the Sloan Digital Sky Survey. A total
of 266,433 background quasars were searched for the presence of absorption
systems in their spectra. The continuum modelling for the quasar spectra was
performed using a mean filter. A pseudo-continuum derived using a median filter
was used to trace the emission lines. The absorption system catalogue contains
39,694 Mg II systems detected at a 6.0, 3.0$\sigma$ level respectively for the
two lines of the doublet. The catalogue was constrained to an absorption line
redshift of 0.35 $\le$ z$_{2796}$ $\le$ 2.3. The rest-frame equivalent width of
the $\lambda$2796 line ranges between 0.2 $\le$ W$_r$ $\le$ 6.2 $\AA$. Using
Gaussian-noise only simulations we estimate a false positive rate of 7.7 per
cent in the catalogue. We measured the number density $\partial
N^{2796}/\partial z$ of Mg II absorbers and find evidence for steeper evolution
of the systems with W$_r \ge$ 1.2 $\AA$ at low redshifts (z$_{2796}$ $\le$
1.0), consistent with other earlier studies. A suite of null tests over the
redshift range 0.5 $\le$ z$_{2796}$ $\le$ 1.5 was used to study the presence of
systematics and selection effects like the dependence of the number density
evolution of the absorption systems on the properties of the background quasar
spectra. The null tests do not indicate the presence of any selection effects
in the absorption catalogue if the quasars with spectral signal-to-noise level
less than 5.0 are removed. The resultant catalogue contains 36,981 absorption
systems. The Mg II absorption catalogue is publicly available and can be
downloaded from the link http://srini.ph.unimelb.edu.au/mgii.php
