Background: Identifying precursors to ADHD, which affects up to 5% of children, is crucial for early identification and support. To this end, we used a prospective sample to investigate endogenous attention and activity level in infants with and without an elevated likelihood (EL) for ADHD and investigated associations with ADHD traits at 3-years. EL status was based on a family history of autism and/or ADHD or a diagnosis of neurofibromatosis type 1 (NF1), a genetic condition associated with higher rates of ADHD. Methods: Infants (n = 26 typical likelihood (TL), n = 70 EL-autism, n = 28 EL-ADHD, n = 18 EL-autism + ADHD, and n = 29 NF1) participated in a live puppet task at 10 and/or 14 months. Mixed-effect models compared groups on behaviourally coded Focused Attention and Vigilance (i.e. Sustained Attention) and Movement, which was measured concurrently using an accelerometer to capture activity during these distinct attention states. Finally, we examined the bivariate associations of Attention and Movement, and their interaction, with 3-year parent-reported ADHD traits. Results: Infants with NF1 exhibited less Focused Attention and Vigilance than EL-ADHD [t (1,170) = 3.53, p =.005] or EL-autism [t (1,170) = 5.43, p <.001] infants and this did not differ across age. There were no Attention differences between the EL and TL groups and no Group or age differences in Movement, however Movement did vary by Attention type: (2) = 215.25, p <.001 (Focused Attention < Vigilance < Looking elsewhere). Across the cohort, less Focused Attention at 10 months (rs = -.27, p =.008) was associated with more ADHD traits at 3-years. During Vigilance at 14 months, there was a significant Attention-by-Movement interaction effect (z = 25.65, p <.001), showing that the association between more Vigilance and fewer ADHD traits was most pronounced in infants showing less Movement. Conclusions: Reduced attention was observed from 10 months onwards in those with NF1, but not in those with a familial likelihood of ADHD. Moreover, early focused attention and the ability to modulate activity level by attentive state (i.e. more vigilant, less movement) may be important emerging features associated with later ADHD traits. We consider implications for early detection and early support strategies.