The average age of the menopause in the United Kingdom is 52 but symptoms of the perimenopause (PMP) can start at 40 or earlier. The UK Armed Forces (UKAF) has a young demographic but women serving over the age of 40, potentially with PMP symptoms, has increased. Recent evidence suggested PMP management by Defence GPs was satisfactory but variable. Because of small numbers, Defence GPs have less exposure to the PMP than NHS GPs, a problem exacerbated for Uniformed GPs who have reduced clinical contact time due to military duties. This study aimed to explore Defence GP knowledge and confidence in managing the PMP, identifying factors contributing to disparities. This study used a sequential mixed-methods approach. A survey with self-rating questions and vignettes was undertaken, followed by semi-structured interviews of selectively sampled respondents. 164 out of 542 Defence GPs responded to a survey, which identified reasonable knowledge (91.3% managing cases per guidelines) but reduced confidence (54.3%). Regression analysis identified case exposure to the PMP, education, and gender as significant factors affecting confidence in PMP management. Themes identified from interviews which were felt to affect knowledge and confidence included: Characteristics of the GP (such as gender); lifelong learning and reflective practice; professional exposure to, and awareness of, the PMP; the changing demographic of Defence; an integrated PMP service to provide experiential learning. Case exposure and experiential learning emerged as the main drivers for confidence. For Uniformed GPs, this was affected by limited clinical time, but confidence returned once exposed to just a few PMP cases. Establishing an integrated PMP service in Defence was seen as crucial for improving care as well as providing educational opportunities through experiential learning for Defence GPs.