This paper describes a field experiment with a self-leadership training aimed at helping human service professionals to improve their detached concern and proactivity. Whereas detached concern refers to a state in which human service professionals blend compassion with emotional distance in their interaction with clients, proactivity refers to self-starting and change-oriented behaviour to enhance personal or organizational effectiveness. Based on self-leadership theory, we hypothesized that self-leadership training can enhance detached concern and proactivity. Moreover, based on behavioural plasticity theory, we hypothesized that training participants who are low in occupational self-efficacy are more susceptible to the external influence of self-leadership training, than individuals with higher levels of occupational self-efficacy. We conducted a field experiment with a sample of 223 human service professionals who were either assigned to a training group (n = 94), or a wait-list control group (n = 129). In a 3-month follow-up study, we found that self-leadership training had a positive effect on detached concern and that the intervention was especially effective for participants with low to medium initial levels of pretraining occupational self-efficacy. However, the intervention did not affect participants' level of proactivity. This study adds to the literature on workplace learning by demonstrating the potential of a self-leadership training for the transfer maintenance of newly developed soft skills (i.e., detached concern and proactive behaviour) to the workplace and by pinpointing occupational self-efficacy as an individual predisposition that influences training success.