In the past decade, researchers have argued for translanguaging as an instructional approach that radically re-conceptualizes educators' views on multilingualism as an asset. Teacher education programs play a pivotal role in preparing teachers who enact this vision. Research on teachers' receptivity to translanguaging's epistemological (re)orientation towards mainstreaming multilingualism, however, remains scarce. To redress this gap, this study investigated mainstream preservice teachers' uptake of translanguaging theories while taking an online ESOL methods course required for TESOL certification. Through the analysis of interview data and participants' written reflections and coursework, we explored preservice teachers' beliefs and the evolution of their views on language learning, language, linguistic diversity, and translanguaging over the course of one semester. Findings highlight the challenges and potential benefits of including translanguaging theories in mainstream and TESOL teacher preparation programs' curricula.