There is no college decision that is more thought provoking and guts wrenching than the choice of a major. However, the factors influencing prospective student teachers at teacher training institutions in Eswatini are yet to be known. Thus, the study sought to determine factors influencing the choice of a subject specialisation by prospective Primary Teachers’ Diploma teachers in Eswatini. The study employed a pragmatism paradigm using convergent or triangulation mixed method design. This study involved 16 experts from three teacher-training institutions and a census of 351 prospective teachers who were eventually admitted to these teacher-training institutions for PTD in Eswatini. Data collection involved a triangulation of modified Delphi technique and survey questionnaire. The qualitative data were analysed using thematic analysis while the quantitative data were analysed using descriptive statistics. Findings revealed that student interest, attitude and impression, related exposure, and the nature of subject specialisation were the factors influencing prospective students’ choice of a specialisation at the teacher training institutions. Therefore, the study recommended that tertiary institutions or institutions of higher education should organise student expo which will expose them to different careers and areas of specialisation in each career.