We have obtained [Mg/Fe] measurements for 76.3 per cent of the stars in the Mid-resolution Isaac Newton Telescope Library of Empirical Spectra (MILES) spectral library used for understanding stellar atmospheres and stellar populations in galaxies and star clusters. These abundance ratios were obtained through (1) a compilation of values from the literature using abundances from high-resolution (HR) spectroscopic studies and (2) a robust spectroscopic analysis using the MILES mid-resolution (MR) optical spectra. All the [Mg/Fe] values were carefully calibrated to a single uniform scale, by using an extensive control sample with results from HR spectra. The small average uncertainties in the calibrated [Mg/Fe] values [respectively 0.09 and 0.12 dex with methods (1) and (2)] and the good coverage of the stars with [Mg/Fe] over stellar atmospheric parameter space of the library will permit the building of new simple stellar populations (SSPs) with empirical α-enhancements. These will be available for a range of [Mg/Fe], including both sub-solar and super-solar values, and for several metallicities and ages. These models will open up new prospects for testing and applications of evolutionary stellar population synthesis.