Early intervention is a systematic approach for identifying and co-ordinating support for children up to five years of age with developmental delay. The increasing number of children needing early intervention is leading to longer waiting lists for assessment, treatment and diagnosis. Through a mixed-methods approach, this study examined the impact of waiting lists on parental self-efficacy (PSE). An online survey was completed by 197 mothers of children with special educational needs aged one to seven years. Six mothers later took part semi-structured interviews. The research aimed to establish (a) whether there was a relationship between length of time on waiting lists and PSE, and (b) whether there were specific themes or patterns associated with delayed early intervention and PSE. Results showed no significant relationship between time spent on a waiting list and perceived PSE. The qualitative results, however, found that limited contact and poor quality of interactions alongside a lack of information did reduce perceived PSE.