Prospective memory lapses, which involve forgetting to perform intended actions, afect independent living in older adults. Although memory training using smartphone applications could address them, users are sometimes unaware of available times for training or forget about it, presenting a need for proactive prompts. Existing applications mostly provide time- based prompts and prompts based on users’ cognitive contexts remain an under-explored area. We developed Prompto, a conversational memory coach that detects physiological signals to suggest training sessions when users are relaxed and potentially more receptive. Our study with 21 older adults showed that users were more receptive to prompts and memory training under low cognitive load than under high cognitive load. Interviews and an in-the-wild deployment of Prompto indicated that majority of users appreciated the concept, found it helpful and were likely to respond to its prompts. We contribute towards developing technologies with cognitive context-aware prompting based on users’ physiological readings.