Background: Young children are at risk of not meeting physical activity recommendations. Identifying factors from the first year of life which influence toddlers’ physical activity levels may help to develop targeted intervention strategies. The purpose of this study was to examine early childhood predictors of toddlers’ physical activity across the domains of maternal beliefs and behaviours, infant behaviours and the home environment. Methods: Data from 206 toddlers (53% male) participating in the Melbourne InFANT Program were collected in 2008–2010 and analysed in 2012. Mothers completed a survey of physical activity predictors when their child was 4- (T1) and 9- months old (T2). Physical activity was assessed by ActiGraph GT1M accelerometers at 19- months (T3) of age. Results: One infant behaviour at T1 and one maternal belief and two infant behaviours at T2 showed associations with physical activity at T3 and were included in multivariate analyses. After adjusting for the age at which the child started walking and maternal education, the time spent with babies of a similar age at 4-months (β = 0.06, 95% CI [0.02, 0.10]) and the time spent being physically active with their mother at 9-months (β = 0.06, 95% CI [0.01, 0.12]) predicted children’s physical activity at 19-months of age. Conclusions: Promotion of peer-interactions and maternal-child co-participation in physical activity could serve as a health promotion strategy to increase physical activity in young children. Future research is required to identify other early life predictors not assessed in this study and to examine whether these factors predict physical activity in later life stages.