Background Mental health has become the leading cause of sickness absence in high-income countries. Managers can play an important role in establishing mentally healthy workplaces and coordinating their organization’s response to a mentally ill worker. Objective This pilot study aims to evaluate the feasibility, usability, and likely effectiveness of a newly developed online training program for managers called HeadCoach. HeadCoach aims to build managers’ confidence in supporting the mental health needs of staff and promote managerial behavior most likely to result in a more mentally healthy workplace. Methods In total, 66 managers from two organizations were invited to participate in this pre-post pilot study of HeadCoach, which was made available to managers to complete at their own pace over a 4-week period. Data were collected at baseline and post intervention via an online research platform. The difference in mean scores for each outcome between these two time points was calculated using paired samples t tests. Results Of all the invited managers, 59.1% (39/66) participated in the trial, with complete pre–post data available for 56.4% (22/39) of the participants. The majority of respondents reported positive engagement with the program. During the study period, managers’ knowledge regarding their role in managing mental health issues (P=.01) and their confidence in communicating with employees regarding mental illness (P<.001) significantly increased. In addition, a significant increase was observed from the baseline in managers’ self-reported actions to use strategies to prevent and decrease stress among their team members (P=.02). Conclusions Although caution is needed due to the absence of a control group, preliminary results of this study suggest that HeadCoach could be a feasible, acceptable, and efficient method of training managers in best workplace practices to help support the mental health needs of their staff.