Premised in the field of second/foreign language teacher education, this study examined language teacher cognition (LTC) from a cultural-historical activity theory perspective. Involving three groups of Cambodian pre-service, novice and experienced teachers of English in a tertiary context in Cambodia, its major aim was twofold: to explore teachers’ cognitions about vocabulary instruction and their interrelations with classroom practices and to map such interrelations in its broader contexts such as the institution in which the teachers worked. Data collected from semi-structured and recall interviews, classroom observations, documentation and the researcher’s field notes spanning a six-month period formed the basis of the analysis. In addition to using Strauss’s (1987) grounded approach to explore the teachers’ cognitions and practices, I employed an activity theoretical framework (Leont'ev, 1978, 1981)—‘activity-action-operation’—to tease out three levels of data analysis. Engeström’s (2015) activity systems analysis model was particularly used to map out interconnections among factors that shaped the teachers’ actions and operations at the classroom level and the teachers’ collective activity at the institutional level. In broad strokes, at the action-level, the analysis focused on the teachers’ cognitions (i.e., mental actions), showing vocabulary was perceived as an indispensable component in the language curriculum. The analysis at the operation-level revealed planned and unplanned vocabulary instructional activities were embodied both explicitly and implicitly, in such a mode as embedding in reading and listening comprehension tasks. The teachers’ mental and physical actions were found to be interrelated with the sociocultural contexts of the classroom, the workplace, the teachers’ respective past and present experiences and their future thinking. Moreover, the analysis at the activity-level uncovered a dynamic set of relations within the activity system: pedagogical core, professional/collegial, and institutional spheres. Drawing on the notions of participation in community of practice (Wenger, 1998) and situated and distributed cognition (Lave, 1988; Daniel, 2008), I discussed these findings to show how the teachers’ personal and professional learning trajectories were determined by the nature of the three ‘spheres for learning’. Implications were discussed for the development of vocabulary instructional practice, teacher education and future LTC research.