Children with Autism Spectrum Disorder (ASD) often display atypical sensory reactivity within the first years of life, prior to a diagnosis. This study examined sensory reactivity patterns at 14 months, changes from 14 to 23 months, and later ASD severity at 3 to 5 years of age in children (n = 87) at elevated likelihood of ASD. Results indicated that observed hyporeactivity at 14 months and increases from 14 to 23 months were related to higher ASD severity during the preschool years. Parent report of hyperreactivity at 14 months was associated with higher ASD severity in the RRB domain during the preschool years. Early hypo and hyperreactivity may predict later severity of ASD and aid in subtyping and developing individualized treatments.