Compounds that delay ageing and prevent against age-related diseases in model organisms have potential application in promoting a healthy lifespan in humans. In this work, the roundworm, Caenorhabditis elegans, was exposed to punicalagin, a natural ellagitannin from the pomegranate, and to RO 90-7501, a pharmaceutical compound previously identified to diminish Alzheimer-related Aβ42-induced toxicity. Punicalagin and RO 90-7501 significantly increased mean lifespan of C. elegans. Furthermore, RO 90-7501 significantly increased survival rate under both thermal and oxidative stress conditions. Both compounds inhibit complex I of the electron transport chain, resulting in significantly impaired respiration after 6 h of application, a short-term energy deficit, and a transiently increased release of mitochondrial-derived ROS. The energy-sensing kinase AMPK/AAK 2 in response promotes mitochondrial metabolism following short term metabolic stress, which leads to subsequently compensated or even elevated respiration rates and ATP levels after long-term exposure to both compounds. The initially elevated formation of mitochondrial-derived ROS is sensed by p38 MAPK/PMK-1 and activates the transcription factor Nrf2/SKN-1 and FoxO/DAF-16 as well as the heat shock factor HSF1/HSF-1, which then initiate downstream effects resulting in an adaptive response. This induction of stress defense can be attributed to the upregulation of antioxidant enzymes including superoxide dismutase and/or catalase as well as the improvement of proteostasis and UPR. The adaptive response then terminates the initial ROS signal and in parallel extends lifespan of C. elegans, all of which is abolished by antioxidant supplementation. Taken together, punicalagin and RO 90-7501 are capable of extending lifespan and decelerate ageing in the roundworm C. elegans and may be considered candidate compounds to prevent against ageing and age-related diseases in higher organisms possibly including humans.