Mr. Thong Kosalrith began his professional journey in 1992 as a Mathematics Teacher Trainer at the Municipality Teacher Training Center (MTTC) in Phnom Penh, instructing pre-service teachers in primary-level mathematics and applied science methodologies. Recognizing his expertise, he expanded his impact as a National Teacher Trainer from 1996 to 2001, specializing in orientation and distance training for primary school mathematics. Parallel to his instructional duties, he demonstrated exceptional administrative leadership at MTTC, serving as Deputy Head of the Academic Office from 1996 to 2005 and subsequently as Head of the Academic Office from 2005 to 2017.

Since 2017, he has brought his extensive experience to the Phnom Penh Teacher Education College (PTEC), first as Deputy Head of the Academic Office and National Trainer, and currently as a dedicated Teacher Trainer and National Trainer. A recognized expert in early grade math, he serves as a National Trainer for the Early Grade Mathematics (EGM) package and VVOB. Today, he continues to shape the educational landscape as a Core Expert for the VVOB SMILE program, focusing on specialized mentoring and early grade math teaching methodology to empower the next generation of primary school teachers.