This paper employs Cultural Historical Activity Theory (CHAT) as an analytical heuristic in exploring the internationally prevalent situation of teaching out-of-field. Drawing on interview transcripts, we use CHAT to frame the activity of a young teacher teaching mathematics out-of-field in a small rural K-12 school in Australia. We identify and map some important elements and tensions of out-of-field mathematics teaching, how these tensions change over time and how this activity interacts with in-field teaching. Finally, we consider emotions and identity in relation to the structural elements of CHAT mediational triangles to more fully explore and represent some of the complexities of teaching mathematics out-of-field in a small rural school and implications for responding to the out-of-field phenomenon.