Objectives The benefits of mindfulness-based interventions (MBIs) for stress, depression, and anxiety have been shown in various studies. However, only a few studies have investigated the effects of MBI on positive psychological functioning, and even fewer studies have simultaneously studied positively and negatively valenced variables. Nevertheless, the evaluation of both seems indispensable for understanding mindfulness and its effects on psychological health. Therefore, this randomized controlled trial compared the effects of a home-based, 6-week MBI on positive and negative aspects of three psychological variables. Methods Eighty-seven participants were randomly assigned to an MBI group (n = 40) or a wait-list control group (n = 47). All participants were evaluated in terms of their positive/negative automatic thoughts, self-compassion levels, and use of cognitive emotion regulation strategies before and after the 6-week period. General linear models were used to compare outcomes on positive and negative scales through likelihood ratio tests. Results The MBI group benefited significantly from the intervention. Larger effects of the MBI on positive automatic thoughts (X2(1) = 9.75, p = .001), positive self-compassion (X2(1) = 5.63, p = .02), and “more adaptive” cognitive emotion regulation strategies (X2(1) = 8.99, p = .003) than on their negative counterparts were observed. The effects were moderated by participants’ initial scores for these variables. Conclusions The evaluation of MBI outcomes should consistently include positive and negative aspects of psychological health. In addition, the benefits of MBIs depend on participants’ initial scores for the evaluated variables. Therefore, individual differences before the intervention must be considered in evaluations.