Self-regulation abilities in childhood are predictive of a range of challenges later in life, making it important to identify difficulties in this area as early as possible. Autistic children and those with attention-deficit/hyperactivity disorder (ADHD) often have difficulties with self-regulation, but little is known about the similarities and differences in such abilities across neurodevelopmental conditions. We examined self-regulation using a delay of gratification task in 36-month-old autistic children (n = 20), those showing clinically relevant concerns for ADHD (i.e. ADHD Concerns; n = 24), and Comparison children without these conditions (n = 130); early predictors of self-regulation were also assessed. Both the Autism and ADHD Concerns groups had greater difficulty waiting for a desired snack than the Comparison group. At the longest delay trial (30 seconds), a substantial percentage of autistic children (50%) and those with ADHD Concerns (35%) consumed the snack prematurely, in contrast to only 16% of the Comparison group. Parent-reported temperament-based impulsivity at 18 months and examiner-observed ADHD-like traits at 24 months were associated with increased self-regulation challenges at 36 months, regardless of group. Adjusting for verbal abilities attenuated some of these differences and associations, suggesting that language may be an important mechanism undergirding early self-regulatory abilities. Given possible links between preschool self-regulation and a range of critical functional outcomes, future studies may explore the efficacy of early interventions targeting impulsivity and regulatory behaviors in infants and toddlers at elevated likelihood for developing self-regulation challenges to potentially reduce the impact of these difficulties later in life.