To better understand the critical early years of teaching and teacher identity development, this study explored the mathematics teacher identity of two early career middle school mathematics teachers and the influence of their working communities on its development. Emel and Berk graduated from the same reform-oriented teacher education program and were working in fundamentally different working communities that could be characterized as reform-unsupportive (Emel) and reform-supportive (Berk). They were interviewed three times and were observed extensively in their working communities. Findings revealed two different teacher identities: Emel exhibited a mathematics teacher identity that was more reform-oriented than non-reform-oriented, and Berk exhibited a non-reform-oriented mathematics teacher identity. Their working communities had influenced their mathematics teacher identity in ways that were both theoretically expected (positive effects of supportive conditions and negative effects of unsupportive conditions) and surprising (positive effects of negative conditions and no effects of positive conditions). The findings are discussed in detail with implications for teacher education programs and supports for early career teachers.