Background: Although early intervention (EI) practitioners emphasise the importance of individualised family-centred services for families of children with developmental delay (DD), few empirical studies have evaluated whether EI can improve family quality of life (FQOL). This study aimed to investigate the trajectory of FQOL and its predictors among families of children with DD during the first 12 months of EI. Methods: This study employed a prospective cohort design. Data were collected using structured questionnaires at the placement meeting before the commencement of EI, as well as 3, 6 and 12 months later. We recruited 142 primary caregivers of children with DD in northern Taiwan from March 2015 to August 2016. FQOL was measured using the Mandarin Chinese version of the Beach Centre FQOL Scale. Family resilience (FR) was measured using the Mandarin Chinese version of the FR Assessment Scale. Other independent variables included socio-demographics, type of DD and EI services. Generalised estimating equations were used to perform multivariate analysis. Results: Family quality of life exhibited a significant quadratic trend in the 12 months surrounding EI. The score was the lowest before EI started (89.85), then increased to peak (94.87) at 6 months and then decreased slightly to 92.34 at 12 months. FR followed a significantly increased linear trend during the period. There were significant and positive correlations between FQOL and FR across all time points. Multivariate analysis showed that employed caregivers, FR, sufficient caregiving manpower and satisfaction with marital quality were positively associated with FQOL. Receiving more types of EI services and having fathers who were not Taiwanese nationals were negatively associated with FQOL. Conclusions: Family quality of life and FR increased significantly after receiving EI, revealing the latter's effectiveness. Unemployment, poor marital quality, father being an immigrant, low FR and insufficient family caregiving manpower were associated with lower FQOL, suggesting that these families require more assistance.