Background Adults with attention deficit/hyperactivity disorder (ADHD) or autism spectrum disorder (ASD) face multiple challenges in obtaining and maintaining employment. Aims To identify and describe how adults with ADHD or ASD experienced their ability to work and what factors affected their ability to find a sustainable work situation over time. Methods Individual in-depth interviews were performed with 20 purposively sampled participants with ADHD/ASD. Data were analysed inductively using reflexive thematic analysis. Results Three themes were identified, describing (1) one's own cognitive abilities and challenges, (2) enablement by flexibility and acceptance in the work environment, and (3) accumulated stress that makes the work situation unsustainable over time. Conclusions Over time, a lack of continuity and predictability of support measures caused great stress and exhaustion, with severe consequences for working life and in life in general. Adaptations needed to be individually tailored and include nonoccupational factors. Significance The study shows that adults with ADHD/ASD need long-term interventions that flexibly adapt to individual needs, as they vary over time. The findings suggest that occupational therapists and other health care providers, employers, employment services and other involved agencies should pay a greater deal of attention to stability and predictability over time.