Abstract Background Rates of childhood obesity are increasing globally, with poor dietary quality an important contributory factor. Evaluation of longitudinal diet quality across early life could identify timepoints and subgroups for nutritional interventions as part of effective public health strategies. Objective This research aimed to: (1) define latent classes of mother-offspring diet quality trajectories from pre-pregnancy to child age 8–9 years, (2) identify early life factors associated with these trajectories, and (3) describe the association between the trajectories and childhood adiposity outcomes. Design Dietary data from 2963 UK Southampton Women’s Survey mother-offspring dyads were analysed using group-based trajectory modelling of a diet quality index (DQI). Maternal diet was assessed pre-pregnancy and at 11- and 34-weeks’ gestation, and offspring diet at ages 6 and 12 months, 3, 6-7- and 8–9-years using interviewer-administered food frequency questionnaires. At each timepoint, a standardised DQI was derived using principal component analysis. Adiposity age 8–9 years was assessed using dual-energy X-ray absorptiometry (DXA) and BMI z-scores. Results A five-trajectory group model was identified as optimal. The diet quality trajectories were characterised as stable, horizontal lines and were categorised as poor (n = 142), poor-medium (n = 667), medium (n = 1146), medium-better (n = 818) and best (n = 163). A poorer dietary trajectory was associated with higher maternal pre-pregnancy BMI, smoking, multiparity, lower maternal age and lower educational attainment. Using linear regression adjusted for confounders, a 1-category decrease in the dietary trajectory was associated with higher DXA percentage body fat (0.08 SD (95% confidence interval 0.01, 0.15) and BMI z-score (0.08 SD (0.00, 0.16) in the 1216 children followed up at age 8–9 years. Conclusion Mother-offspring dietary trajectories are stable across early life, with poorer diet quality associated with maternal socio-demographic and other factors and childhood adiposity. The preconception period may be an important window to promote positive maternal dietary changes in order to improve childhood outcomes.