This study compared the effectiveness of a 12-day stress-prevention program (SGS) supplemented by individualized, structured, four-session telephone-coaching to that of an SGS without telephone-coaching in entrepreneurs from the green professions presenting with increased stress levels. All participants went through the SGS before being randomized either to the telephone-coaching group (TC) or to the control group without telephone-coaching (noTC). SGS included four key therapeutic elements: stress-management intervention, relaxation, physical exercise, and balneotherapy. The primary outcome was the current degree of subjectively experienced stress assessed with the Perceived Stress Questionnaire (PSQ) at a 9-month follow-up. Secondary outcomes included burnout symptoms, well-being, health status, sleep disorders, expectation of self-efficacy, depression, anxiety, ability to work, pain, and days of sick leave. Assessments were conducted at baseline, 12 days (end of program), and 1 (start telephone-coaching), 3, 6 (end of telephone-coaching), and 9 months. Data from 103 adults (TC = 51; noTC = 52), mostly fulltime farmers, were available for analysis (mean age: 55.3; 49.1% female). Participants experienced significant immediate improvement in all outcome measurements, which declined somewhat during the first three months after the end of SGS and then remained stable for at least another six months. While within-group changes from baseline to 9 months showed significant improvements at medium to large effect sizes for all target variables (PSQ-total, TC: −13.38 (±14.98); 95%-CI: (−17.68; −9.07); noTC: −11.09 (±14.15); 95%-CI: (−15.11; −7.07)), no statistically significant differences were found between the groups at any time and for any target variable (between-group ANCOVA for PSQ-total at 9 months, parameter estimator for the group: −1.58; 95%-CI: (−7.29; 4.13)). The stress-prevention program SGS is a feasible, effective, and practical way to reduce perceived stress and improve participants’ resources. Four subsequent telephone-coaching sessions do not seem to contribute to a further improvement in the results.