Objectives To analyse possible changes in oral health-related quality of life (OHRQoL) before and after dental treatment under dental general anaesthesia (DGA) among Austrian preschool children. Methods A consecutive sample of 89 parents of children aged 2 to 5 years, suffering from early childhood caries (ECC) and scheduled for DGA, were recruited from two locations in Austria (Vienna and Salzburg). Parents self-completed the German version of the ECOHIS before (baseline) and 4 weeks (T4) after their child’s dental treatment. The ECOHIS consists of 13 questions and is divided into two main parts, namely, the child impact section (9 items) and the family impact section (4 items). Results A total of 80 children (89%) completed a sufficient number ECOHIS questions at baseline and the follow-up assessment after 4 weeks. “Pain in the teeth, mouth, and jaws” and “difficulty eating some foods” from the child section and parents’ ratings of “feeling upset” and “guilty” were the most frequently reported impacts at baseline. The ECOHIS total score decreased significantly from a mean of 14.60 to 9.89 (p < 0.001) after DGA treatment, revealing a large effect size for the child (0.8) section, family (0.6) section, and the total score (0.8). Parents rated their child’s overall and oral health significantly higher after the DGA treatment (p < 0.001). Conclusions Significant improvements in oral health-related quality of life were observed 4 weeks after DGA in children suffering from ECC. Clinical relevance ECC has an impact on OHRQoL. Rehabilitation under general anaesthesia makes a sustainable improvement.