Omega-3-polyunsaturated-fatty-acids were suggested against cognitive dysfunctions and conversion to psychosis. However, a recent multicenter trial found no effect in reducing conversion rates in individuals at risk of developing schizophrenia. Patients' genetic heterogeneity and the timing of treatment might influence omega-3 efficacy. Here, we addressed the impact of omega-3 early treatment in both mice and human subjects with a 22q11.2 genetic hemi-deletion (22q11DS), characterized by cognitive dysfunctions and high penetrance of schizophrenia. We first tested the behavioural and cognitive consequences of adolescent exposure to normal or omega-3-enriched diets in wild-type and 22q11DS (LgDel/+) mice. We then contrasted mouse data with those gathered from sixty-two patients with 22q11DS exposed to a normal diet or supplemented with omega-3 during pre-adolescence/adolescence. Adolescent omega-3 exposure had no effects in wild-type mice. However, this treatment ameliorated distractibility deficits revealed in LgDel/+ mice by the Five Choice Serial Reaction Time Task (5CSRTT). The omega-3 improvement in LgDel/+ mice was selective, as no other generalized cognitive and non-cognitive effects were evident. Similarly, omega-3-exposed 22q11DS patients showed long-lasting improvements on distractibility as revealed by the continuous performance test (CPT). Moreover, omega-3-exposed 22q11DS patients showed less risk of developing an Ultra High Risk status and lower conversion rate to psychosis. Our convergent mouse-human findings represent a first analysis on the effects of omega-3 early treatment in 22q11DS. The beneficial effects in attentional control and transition to psychosis could support the early use of omega-3 supplementation in the 22q11DS population.