Quantitative MRI provides objective non-invasive biomarkers for muscle pathology in muscular dystrophy disorders. In this work we show that MR biomarkers for muscular fat infiltration and atrophy accurately reflect clinical outcomes for disease severity and physical capacity in myotonic dystrophy type 1 (DM1) patients. Furthermore, we found that 37% of the variation in fat infiltration in DM1 patients was explained by age. Interestingly, an additional 9.7% of the variation in fat infiltration was associated with the over life time increase in the DMPK CTG repeat length, i.e. the genetic defect causing DM1.