Background: Governments are promoting the importance of maintaining cognitive health into older age to minimize risk of cognitive decline and dementia. Older people with amnestic mild cognitive impairment (aMCI) are particularly vulnerable to memory challenges in daily activities and are seeking ways to maintain independent living. Objective: To evaluate the effectiveness of memory groups for improving memory strategies and memory ability of older people, especially those with aMCI. Methods: 113 healthy older adults (HOA) and 106 adults with aMCI were randomized to a six-week memory group or a waitlist control condition. Outcome was evaluated through knowledge and use of memory strategies, memory ability (self-report and neuropsychological tests), and wellbeing. Assessments included a six-month follow-up. Results: Using intention to treat analyses, there were intervention effects for HOA and aMCI groups in strategy knowledge (HOA: η2 = 0.20; aMCI: η2 = 0.06), strategy use (HOA: η2 = 0.18; aMCI: η2 = 0.08), and wellbeing (HOA: η2 = 0.11; aMCI: η2 = 0.05). There were also intervention effects in the HOA group, but not the aMCI group, in self-reported memory ability (η2 = 0.06) and prospective memory tests (η2 = 0.02). By six-month follow-up, gains were found on most HOA outcomes. In the aMCI group gains were found in strategy use, and by this stage, gains in prospective memory were also found. Conclusion: Memory groups can engage older people in techniques for maintaining cognitive health and improve memory performance, but more modest benefits are seen for older adults with aMCI.