The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) has been measuring the solar UV irradiance since October 1991, extending from the near maximum of solar cycle 22 to the declining phase of solar cycle 23. Among its daily scientific products is the MgII core-to-wing ratio, a solar activity index based on spectral irradiance measurements of the strong MgII absorption feature near 280 nm. The dimensionless MgII index is ratio of the core irradiance to that of the quasi-continuum in the nearby wings, but whose precise definition varies from one experiment to another. Accordingly, it is relatively insensitive to instrumental changes since these tend to be common to both irradiances. Despite differences in instruments and algorithms, a high level of correlation among individual MgII indices has been found. These properties have led Viereck et al. (2004) and others to construct a MgII composite index, extending from 1978 to the present. Produced only a few days after measurements are taken, the SUSIM MgII index (version V21r2), at 13+ years, is currently the longest instrumental MgII time series available. We present aspects of the SUSIM MgII algorithm and we compare it with various solar spectral irradiances, other versions of the MgII index, and with the F10.7 index commonly used in modeling studies.