The present study investigated the effects of harvesting time on the physicochemical properties, antioxidant activity, fatty acid composition, and phenolic compounds of peanut kernels. The moisture content (air-dried basis) of peanut kernels was determined between 4.47% (September 15, 2019) and 7.93% (October 6, 2019), whereas the oil contents changed from 45.95% (October 6, 2019) to 49.25% (September 22, 2019). The total carotenoid, chlorophyll, and phenolic contents were low throughout the harvest, showing differences depending on the harvest time. Total phenolic content changed from 0.28 mg GAE/L (September 29, 2019) to 0.43 mg GAE/L (September 8, 2019), whereas the antioxidant activity varied from 4.42% (August 25, 2019) to 4.70% (September 1, 2019). The dominant fatty acids were palmitic, oleic, and linoleic acids, depending on the harvest time, followed by stearic, behenic, arachidic, and linolenic acids. The (+)-catechin content ranged from 2.17 mg/L (September 8, 2019) to 5.15 mg/L (September 1, 2019), whereas 1,2-dihydroxybenzene content changed between 2.67 mg/L (October 6, 2019) and 5.85 mg/L (September 29, 2019). The phenolic compound content fluctuated depending on the harvest time. The results showed that peanut kernel and oil had distinctive phenolic profiles and fatty acid contents. The findings of the present study may provide information for the best time to harvest peanut to achieve its maximum health benefits.