Understanding numerical magnitude is critical to the development of mathematics proficiency, and math interventions targeting magnitude knowledge have been shown to improve outcomes for students with math learning difficulties across grade levels. While recent studies have found that growth in magnitude knowledge mediates fractions intervention outcomes, similar analyses have not been conducted in the context of early numeracy interventions. This secondary analysis examined whether and to what degree gains in magnitude understanding explained overall intervention gains for kindergarten students (n = 1,251) receiving a validated Tier 2 early math intervention, ROOTS. Results of an indirect-effects mediation analysis were consistent with a partial mediation effect across proximal and distal measures of mathematics proficiency. Findings add to a growing literature base highlighting the importance of magnitude understanding to overall mathematics proficiency in the early grades, and indicate that whole number magnitude understanding may be an "active ingredient" driving early mathematics intervention outcomes. [This paper will be published in "Learning Disabilities Research & Practice."]