Mindfulness training appears to reduce stress and distress, but little is known about whether it results in changes that can be observed by colleagues, family, or friends or its appropriateness as a workplace stress management intervention for a large and distributed public sector workforce. This study evaluated a pilot 5-week Mindfulness at Work Program (MaWP) for acceptability, feasibility, and efficacy in relation to stress and related mental health and productivity problems for public sector employees. A parallel group randomized controlled trial compared the MaWP intervention (n = 20) with an information-only control (n = 100). Exploratory qualitative and quantitative methods were used to assess changes observed by informants (n = 63). Results suggest a high degree of acceptability, although location and inflexible work schedules presented feasibility obstacles. Compared with the control, the primary outcome of mindfulness improved for MaWP participants (d = 0.57, p < 0.001), as did perceived stress (d = 0.97, p < 0.001), psychological distress (d = 0.61, p < 0.001), health-related quality of life (d = 0.51, p = 0.002), and social functioning (d = 0.08, p = 0.019). All secondary outcomes were at least partly mediated by changes in mindfulness. The intervention thus appears to have potential merit as a workplace intervention for public sector employees across a range of outcomes. Obtaining informant observations was feasible and while qualitative analyses indicated positive changes that supported self-reported outcomes, quantitative analyses returned ambiguous results. A seven-item scale adapted from a popular self-report mindfulness scale for use by informants showed promise, but further work is needed to establish validity, reliability, and scalability of this method of assessing observable changes following mindfulness training.