The ability to sustain attention is a major achievement in human development and is generally believed to be the developmental product of increasing self-regulatory and endogenous (i.e., internal, top-down, voluntary) control over one’s attention and cognitive systems [1, 2, 3, 4, 5]. Because sustained attention in late infancy is predictive of future development, and because early deficits in sustained attention are markers for later diagnoses of attentional disorders , sustained attention is often viewed as a constitutional and individual property of the infant [6, 7, 8, 9]. However, humans are social animals; developmental pathways for seemingly non-social competencies evolved within the social group and therefore may be dependent on social experience [10, 11, 12, 13]. Here, we show that social context matters for the duration of sustained attention episodes in one-year-old infants during toy play. Using head-mounted eye tracking to record moment-by-moment gaze data from both parents and infants, we found that when the social partner (parent) visually attended to the object to which infant attention was directed, infants, after the parent’s look, extended their duration of visual attention to the object. Looks to the same object by two social partners is a well-studied phenomenon known as joint attention, which has been shown to be critical to early learning and to the development of social skills [14, 15]. The present findings implicate joint attention in the development of the child’s own sustained attention and thus challenge the current understanding of the origins of individual differences in sustained attention, providing a new and potentially malleable developmental pathway to the self-regulation of attention.