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Simple Summary: Endophytic fungi in pasture grasses help the plant resist unfavourable biotic and abiotic conditions. Despite some well-documented aspects of how these fungi confer resistance to insects, some mechanisms are not well known. In this work, we measured the effects of such endophytes on aphids, an important pest group. We measured aphid feeding activity and life-history effects to further understand the mode of action of endophyte–perennial ryegrass systems. We were able to demonstrate that, in some cases, fungal endophytes significantly deterred feeding, but in other cases, feeding was not affected, and yet high mortality was observed. This suggests a complex scenario of interactions between compounds in the endophyte–perennial-ryegrass–aphid system. Our study shows that fungal endophytes have the potential to improve sustainable aphid control by decreasing the use of insecticide sprays. The bird cherry-oat aphid, Rhopalosiphum padi (L.), is an economically significant pest of pasture grasses, the latter being capable of hosting several fungal endophyte–perennial ryegrass symbiota rich in alkaloids and toxic to vertebrates and invertebrates. Measuring aphid feeding behaviour can provide insights into the effectiveness and mode of action of different fungal endophytes. This study investigated the effects of different Epichloë–perennial ryegrass symbiota on the feeding behaviour of R. padi using the electrical penetration graph technique while also assessing the aphid life history. In most cases, endophytes had significant feeding deterrence and paired fecundity and mortality effects. But, in some instances, endophytes with the highest aphid mortality did not significantly deter feeding, suggesting a more complicated scenario of interactions between the relative concentration of metabolites, e.g., host plant defence response metabolites and alkaloids, and/or physical changes to leaf morphology. Overall, this study sheds light on the mode of action of Epichloë endophytes against aphids and highlights the importance of Epichloë–perennial ryegrass symbiota in the management of insect pests such as aphids in pasture-based grazing systems. [ABSTRACT FROM AUTHOR]