Despite the recent growth of mobile instant messenger (MIM), research into the correlation between English learners' learning gains, learning engagement, and how this engagement helps develop authentic relationships remains underexplored. This mixed-method study involved 222 college English learners to investigate their engagement in team-based mobile learning (TBML) assisted by "WhatsApp" as an MIM and its relationship with their learning gains. In addition, it was designed to document the social construction of existential and relational authenticity between teacher and students. The study collected data on students' online learning engagement through an online survey, and their learning achievement was measured by the course final examination. Grounded in thematic discourse analysis, the study collected qualitative data from the archives of "WhatsApp" group chats. Retrospective reflection was also used to triangulate the findings. The findings show the value of MIM to develop strong engagement and authentic relationships in socio-constructivist learning as the precursor to learning achievement.