With the prevalent sentiments on the effects of Mother Tongue Based – Multilingual Education (MTB-MLE), this study investigated the lived experiences of the Key Stage 1 teacher in an elementary school setting which focused on outcomes and challenges to capture the invariant structure of lived experiences common to all study participants within Schools Learning Cluster 3 (SLC-3) in the Zamboanga Sibugay Division. The study participants were only eight (8) teachers who had at least five years of experience in teaching about Mother Tongue – Based Multilingual Education (MTB-MLE) under the K to 12 Basic Education Curriculum. This research endeavor employed both qualitative and quantitative designs. It is qualitative in the sense that it utilized Colaizzi’s phenomenological research design. Eventually, it is also quantitative because it investigated the significant difference between the Grade 3 pupils’ academic performance of central and non-central schools. Results revealed that after merging all reflected themes the whole structure of the phenomenon “MTB-MLE Impacts, Challenges and Key Stage 1 teachers’ Coping Mechanism towards Implementation” and it was also found that there was a significant difference between the Grade 3 pupils’ academic performance of central and non-central schools. This concluded that despite the MTB-MLE impressive outcomes, Key Stage 1 teachers experienced challenges in implementation.