Research demonstrates the importance of nutrition for early brain development. Few studies have examined the effectiveness of multiple micronutrient powders (MNP) on child development. This study examined the impacts of home fortification with MNP on motor and mental development, executive function and memory of children living in Bihar. This two-arm cluster-randomised effectiveness trial selected seventy health sub-centres to receive either MNP and nutrition counselling (intervention) or nutrition counselling alone (control) for 12 months. Front-line health workers delivered the intervention to all households in study communities with a child aged 6–18 months. Data were collected using cross-sectional surveys at baseline and endline by selecting households from intervention (baseline, n 2184; endline, n 2170) and control (baseline, n 2176; endline, n 2122) communities using a two-stage cluster-randomised sampling strategy. Children in the intervention group had a significantly larger improvement from baseline to endline compared with those in the control group on scores for motor and mental development (Cohen’s d, motor=0·12; 95 % CI 0·03, 0·22; mental=0·15; 95 % CI 0·06, 0·25). Greater impacts of MNP on motor and mental development were observed in children from households with higher stimulation scores at baseline compared with those with lower stimulation (Cohen’s d, motor=0·20 v. 0·09; mental=0·22 v. 0·14; Pinteraction<0·05). No significant treatment differences were seen for executive function or memory. Home fortification with MNP through the existing health infrastructure in Bihar was effective in improving motor and mental development and should be considered in combination with other child development interventions such as stimulation.