The impact of native (NS) and non-native speaker (NNS) identities on second or foreign language teachers' cognition and practices in the classroom has mainly been investigated in ESL/EFL contexts. Using complexity theory as a framework, this case study attempts to fill the gap in the literature by presenting a foreign language teacher in the United States who teaches French as a NS and German as a NNS teacher, at the college level. Specifically, the study explores the interface between NS/NNS identities, teacher cognition, practice, and professional identity. The retrodictive qualitative analysis of semi-structured interviews and classroom observations reveals that the participant teacher's NS French and NNS German identities influenced her teacher cognition, specifically in beliefs about teaching grammar. In addition, previous language learning experiences affect current decision-making processes in teaching. In terms of teaching practice, the dual NS French and NNS German identity affects teaching practice in the formal areas of language, target culture knowledge and awareness, teaching style, and perceptions about language varieties. Professional identity is construed here as being the mediator of target second language (L2) cultures in the classroom. The implications of teacher cognition as a complex system for L2 teaching and teacher education are discussed.