Teacher self-efficacy (TSE) is among the most-studied constructs in teacher education and resists change after the early stages of development, particularly during professional experience. This study investigates the sources of early-career TSE longitudinally, which few previous studies have done. Two phases of one-on-one interviews were completed with pre-service teachers after the first (n = 18) and final (n = 13) professional experience placements across the final two pre-service years. The data were analysed thematically and are reported according to Bandura's four sources of self-efficacy. Mastery experience was the predominant source across the study, followed by emotional states. Vicarious experience and social persuasion were less reported. Both variations in how each source influenced TSE and interactions between different sources across the study were apparent. Implications and limitations are discussed.