Background: Gardening has been reported as being beneficial for mental well-being for vulnerable populations since 2000. However, little is known concerning its role in the general population. Therefore, the aim of the present study was to investigate the relationship of gardening and mental health in adults in a countrywide and population-based setting. Methods: Data was retrieved from and analysed in the Scottish Health Survey, 2012–2013. Information on demographics, lifestyle factors, gardening engagement, and adult mental health by General Health Questionnaire was obtained by household interview. Statistical analyses including chi-square test, t-test and survey-weighted logistic and multi-nominal regression modelling were performed. Results: Of 9709 Scottish adults aged 16–99, 5 531 (57.0%) people did not do any gardening or building work in the last four weeks. A total of 888 (9.2%) people reported poor self-rated health. Gardening was associated with adult mental health in people both with or without heart conditions including ability to concentrate, feeling playing a useful part in things, feeling capable of making decisions, thinking of self as worthless, feeling reasonably happy, etc. Conclusion: General adults with or without heart conditions could benefit from engaging with gardening or building work. Future public health programmes promoting such activity should be encouraged in order to optimise adult mental health.