COVID-19 pandemic has had a great influence on educational systems all over the world. The changes which occurred during the last year and a half were related to adapting the teaching-learning process to the online environment, to students’ and teachers’ behaviour regarding the way they got involved in educational activities, and to the lack of proper training for both categories. The mastery learning model can be adapted to the new teaching and learning conditions, being a great advantage, especially for students who encounter different difficulties in learning mathematics. The purpose of this qualitative research was to determine how effective the implementation of a mastery learning strategy is for both in-class and online math lessons. The implemented mastery learning strategies found a fertile ground in this new learning environment and have revealed that math anxiety and test anxiety were reduced due to the constant use of formative assessments and to the opportunity to revise contents that had not been previously understood.