Emotion dysregulation is a common challenge for autistic individuals, yet research examining early emotion regulation processes in autism is lacking. The present study examined negative emotion expression and parent-child co-regulatory processes in 18-month-old younger siblings of autistic children (children with an "elevated likelihood" (EL) of autism) with diverse outcomes: autism, language delay without autism (EL-LD), and no delay/diagnosis (EL-ND). Thirty-nine toddlers were videotaped at home with a parent. Negative emotion and parent co-regulatory responses were coded from the video. Results indicated that autistic toddlers exhibited more high-intensity negative emotion than EL-ND toddlers. The likelihood of negative emotion continuing once initiated was higher for autistic and EL-LD toddlers. Parental co-regulation strategy use was similar across groups. Parental co-regulation reduced the likelihood of continued negative emotion, although the effect appeared somewhat weaker for autistic toddlers. This research corroborates evidence that increased risk for heightened and prolonged negative emotion starts early in autistic children and, to a lesser extent, in EL-LD children. Parents of all children were highly responsive, but coregulatory responses may be less effective at reducing negative emotions for autistic children. While more research is needed, the present study contributes to our understanding of the unique emotional experiences of autistic toddlers.