The present study examined individual differences in the development of sustained attention across toddlerhood, as well as how these individual differences related to the development of language and sleep. Toddlers (N = 314; 54% male) were assessed at 30, 36, and 42 months using multiple measures of attention, a standardized language assessment, and actigraphic measures of sleep. Toddlers were 80% White. Family socioeconomic status (SES) was calculated using the Hollingshead Four Factor Index and ranged from 13 to 66 (M = 47.59, SD = 14.13). Aims were (a) to examine associations between measures of attention across situations, informants, and time; (b) to consider the independent and interactive effects of language and sleep on attention; and (c) to test potential bidirectional associations between sleep and attention. Findings showed attention measures were stable across time but were only weakly linked with each other at 42 months. Attention was consistently linked with language. More variable sleep and longer naps were associated with less growth in sustained attention across time. Nighttime sleep duration interacted with language in that sleep duration was positively associated with attention scores among toddlers with less advanced language, even when SES was controlled. The findings describe an understudied aspect of how sustained attention develops, involving the main effect of consistent sleep schedules and the interaction effect of amount of sleep and child language development. These findings are relevant to understanding early childhood risk for developing attention problems and to exploring a potential prevention target in family sleep practices.