We investigated the effects of increasing physical activity (PA) and reducing sedentary behaviour (SB) of mental health professionals on their attitudes towards and practices in recommending more PA and less SB to their clients. A 4-week pre-post intervention trial was conducted involving 17 mental health professionals. The participants who increased their own physical activity during the intervention increased the frequency of recommending more PA (p = 0.009) and less SB (p = 0.005) to their clients. A relatively simple, low-cost intervention, consisting of group behaviour change counselling, goal setting and positive feedback, may improve the practices of mental health professionals.Supplemental data for this article is available online at https://doi.org/10.1080/01612840.2021.1972189 .