Memories encoded during infancy are often rapidly forgotten. This accelerated memory decline is known as infantile amnesia, and is hallmarked by the inability of natural cues to induce the retrieval of early-life memories later in life. Remarkably, infant memories in rodents are not lost. Artificial stimulation of infant memory ensembles can drive according behavior, leading to the hypothesis that infant memories persist through time albeit in a latent state. In our study, we aim to unravel under which circumstances “forgotten” infant memory ensembles can be reactivated by natural cues in adulthood and by which mechanisms this is accompanied by long-lasting memory reinstatement. Therefore, we permanently tag hippocampal ensembles active at encoding of a contextual fear memory during infancy, to record activity dynamics of the infant memory ensemble through calcium imaging in adulthood, and manipulate its activation with optogenetics upon behavioral protocols leading to memory reinstatement. Our results show that stable memory ensembles persist from infancy to adulthood. They are reactivated during experiences related to the original memory and during offline brain activity. The integration of external reminders linking context to content in a non-associative form is necessary for reinstatement, whereas a reminder of the emotional state is not sufficient. Last, the activation of the latent infant memory trace is required for reinstating the “forgotten” memory, but not for its recall. Together, these results shed new light on the biology of seemingly forgotten infant memories, and might inform new strategies to prevent the reinstatement of memories related to traumatic infant events.