This article explores how high school teachers construct their stock of knowledge and its impact on their professional identity. Through in-depth interviews with teachers from both public and private institutions in Guadalajara, Mexico, the study employs Alfred Schütz’s phenomenological sociology to analyze the integration of disciplinary and educational knowledge. Findings indicate that teachers live through tensions between their original professions and their educational responsibilities, shaping their identities through social interactions and experiential learning. The study further reveals that institutional support and continuous professional development play a pivotal role in facilitating this integration. By highlighting the dynamic nature of teacher identity formation, this research highlights the need for interdisciplinary training programs that bridge subject-matter expertise and pedagogical skills. Future studies should explore adaptive learning models and mentorship programs that enhance the transition for professionals entering the teaching field.