This paper documents the development of the Mosaic Readiness Index, a score that is reported for individuals who take the high school version of Mosaic by ACT®: Social Emotional Learning Assessment (hereafter referred to as the Mosaic assessment). The Mosaic Readiness Index is designed to predict high school academic outcomes--namely high school coursework, high school grades, and ACT test scores. Compared to the Mosaic assessment scores for individual constructs, the Mosaic Readiness Index more strongly predicts academic readiness because this index optimizes its prediction using item-level data. This paper describes the (a) rationale behind the index, (b) sample of data and statistical methods used for estimating the index's scoring parameters, and (c) results that show how well the index predicts academic readiness. The authors also document the development of the percentile ranks and score levels that are reported for the index and compare average scores for different groups of students.