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Tropospheric ozone is an abiotic stress of increasing importance in the context of global climate change. This greenhouse gas is a potent phytotoxic molecule with demonstrated negative effects on crop yield and natural ecosystems. Recently, oxidative stress has been proposed as a mechanism that could regulate the interaction between cool‐season grasses and Epichloë endophytes. We hypothesized that exposure of Lolium multiflorum plants, hosting endophytes to an ozone‐polluted environment at different ontogenetic phases, would impact the trans‐generational dynamics of the vertically transmitted fungal symbiont. Here, we found that the ozone‐induced stress on the mother plants did not affect the endophyte vertical transmission but it impaired the persistence of the fungus in the seed exposed to artificial ageing. Endophyte longevity in seed was reduced by exposure of the mother plant to ozone. Although ozone exposure did not influence either the endophyte mycelial concentration or their compound defences (loline alkaloids), a positive correlation was observed between host fitness and the concentration of endophyte‐derived defence compounds. This suggests that fungal defences in grass seeds were not all produced in situ but remobilized from the vegetative tissues. Our study reveals ozone trans‐generational effects on the persistence of a beneficial symbiont in a host grass. Mother plant exposure to ozone did not affect the growth and the transmission of fungal endophytes into host seeds. Seed alkaloid concentration correlated with endophyte mycelial concentration and host fitness. However, ozone‐induced stress on mother plants did impair the longevity of the endophytes in the seeds. [ABSTRACT FROM AUTHOR]