Many autistic people and people with non-spectrum neurodevelopmental disabilities (e.g. intellectual disability) report feeling lonely, which can negatively impact their well-being. There is little longitudinal research, however, tracking changes in how autistic people experience, conceptualize, and cope with loneliness throughout their lives. A longitudinal sample of 114 people, which included autistic participants and participants with neurodevelopmental disabilities, characterized experiences of loneliness, perceptions of other people's loneliness, and strategies used to cope with loneliness from childhood to adulthood. Level of loneliness and coping strategies were coded from Autism Diagnostic Observation Schedule Modules 3 and 4 protocol forms. Autism Diagnostic Observation Schedule Loneliness Ratings were correlated across time and increased from adolescence to young adulthood. The most common loneliness coping strategies were Behavioral Distraction (e.g. watching TV) and Instrumental Action (e.g. seeking social contact), which were both used by more people in adulthood than childhood. Those who used Behavioral Distraction and a greater number of coping strategies had higher Autism Diagnostic Observation Schedule Loneliness-Self Ratings (i.e. were lonelier) during adolescence and adulthood. Findings highlight adulthood as a particularly vulnerable time for loneliness and indicate a need for more support and social opportunities for autistic adults and adults with neurodevelopmental disabilities who wish to make more social connections.