Objectives: This review informed development of the first national Physical Activity (PA) Guidelines for Older Australians with Mild Cognitive Impairment (MCI) or Subjective Cognitive Decline (SCD) (http://www.dementiaresearch.org.au/images/dcrc/output-files/1567-pa_guidelines_for_mci_or_scd_full_report_final.pdf). These guidelines are directed at healthcare professionals and aim to encourage older adults with SCD/MCI to engage in PA to enhance cognitive, mental and physical health. Design: A narrative review was undertaken to inform the guideline adaptation process. Methods: A systematic search of existing PA guidelines for older adults was performed and evaluated using the Appraisal of Guidelines for Research and Evaluation II Instrument. The guideline assessed as most appropriate was adapted to the population with SCD/MCI using the Guideline Adaptation Resource Toolkit, supported by the narrative review. Results: The search for existing PA guidelines for older adults yielded 22 guidelines, none of which specifically considered older adults with SCD/MCI. The Canadian Physical Activity Guidelines for Older Adults were selected for adaptation to the population with SCD/MCI. The narrative review found 24 high-quality randomised controlled trials and 17 observational studies. These supported the four guideline recommendations that address aerobic PA, progressive resistance training, balance exercises and consultation with healthcare professionals to tailor PA to the individual. Conclusions: This review found evidence to support the four guideline recommendations. These recommendations provide specific guidance for older adults with SCD/MCI, their families, health professionals, community organisations and government to obtain benefits from undertaking PA. The review also highlights important future research directions, including the need for targeted translation and implementation research for diverse consumers.