The complementary feeding period is an important stage of child development. The study aim was to develop an index reflecting the degree of adherence to complementary feeding guidelines, evaluate its convergent validity, and explore associations with socio-demographic factors and dietary pattern scores in childhood. Data were analyzed from the Avon Longitudinal Study of Parents and Children (n = 6065) using parent-completed dietary questionnaires at 6 mo of age, socio-demographic information, and dietary patterns derived by principal component analysis at age 3 y. The Complementary Feeding Utility Index (CFUI) consists of 14 components: breastfeeding duration, feeding to appetite, timing of introduction to solids, exposure to iron-rich cereals, fruit and vegetable intake, exposure to high-fat/-salt/-sugar foods including sugary drinks, food texture, and meal/snack frequency. Regression analyses were undertaken to investigate associations between index scores, socio-demographic factors, food and nutrient intakes, and dietary pattern scores at age 3 y. Milk and food intake at 6 mo and nutrient intake at 8 mo of age varied across quintiles of index score in largely the expected directions. Associations were found among index score, maternal age, education, social class, maternal smoking history, and prepregnancy BMI. After adjustment for socio-demographic factors, the index score was associated with "processed" [β = -0.234 (95% CI = -0.260, -0.209)] and "healthy" [β = 0.185 (95% CI = 0.155, 0.215)] dietary pattern scores at age 3 y. The CFUI is able to discriminate across food intake, nutrient intake, and socio-demographic factors and is associated with later dietary patterns.