Applied Relaxation (AR) is an established behavioral mental health intervention, but its efficacy in real life contexts remains unclear. Using randomized controlled trial data, we examined whether AR can effectively reduce mental health problems in daily life. A sample of 277 adults with increased psychopathological symptoms but without 12-month DSM-5 mental disorders at study entry was randomly assigned to an intervention group receiving AR training (n = 139) and an assessment-only control group (n = 138). Ecological momentary assessments were used to assess psychological outcomes in daily life over a period of seven days at baseline, post, and 12-month follow-up, respectively. Multilevel analyses indicated that all psychopathological symptoms decreased more in the intervention group than in the control group from baseline to post (range β = -0.31 for DASS-depression to β = -0.06 for PROMIS-anger). However, from post to follow-up, psychopathological symptoms decreased more in the control group than in the intervention group, so that only the intervention effects for PROMIS-depression (β = -0.10) and PROMIS-anger (β = -0.09) remained until follow-up. Moreover, positive affect (β = 0.19), internal control beliefs (β = 0.15), favorable coping (β = 0.60), and unfavorable coping (β = -0.41) improved more in the intervention group than in the control group, and these effects were mostly maintained in the long term. Some effects were stronger among women, older individuals, and individuals with higher initial symptoms. These findings suggest that AR can effectively reduce mental health problems in daily life. Trial registration. The trial has been registered at ClinicalTrials.gov (NCT03311529).