Maternal depression and anxiety have been proposed to increase the risk of adverse outcomes of language development in the early years of life. This study investigated the effects of maternal depression and anxiety on language development using two approaches: (i) a categorical approach that compared lexical abilities in two groups of children, a risk group (mothers with clinical-level symptomatology) and a control non-risk group, and (ii) a continuous approach that assessed the relation between individual mothers’ clinical and subclinical symptomatology and their infants’ lexical abilities. Infants’ lexical abilities were assessed at 18 months of age using an objective lexical processing measure and a parental report of expressive vocabulary. Infants in the risk group exhibited lower lexical processing abilities compared to controls, and maternal depression scores were negatively correlated to infants’ lexical processing and vocabulary measures. Furthermore, maternal depression (not anxiety) explained the variance in infants’ individual lexical processing performance above the variance explained by their individual expressive vocabulary size. These results suggest that significant differences are emerging in 18-month-old infants’ lexical processing abilities, and this appears to be related, in part, to their mothers’ depression and anxiety symptomatology during the postnatal period.