He won the regional swimming competitions and went on to win medals at the University, National, SEAP, and Asian Games.

In his freshman year, he participated in the Yangon All Clubs Swimming Competition and broke the national records in 100m, 200m, 400m, 800m and 1500m freestyle.

In 1960, he participated in the Rome Olympics, he competed in the 1500 metre freestyle event and finished in 20th place but did not advance to the final.

[2] In 1961 SEAP Games Yangon, he won gold medals in 200m, 400m and 1500m individual free style swimming and broke the record for 4 x 100 m freestyle relay.

Tin Maung Ni participated in 1964 Tokyo Olympic and competed in 400m and 1500m freestyle and finished 6th and 4th in the heats, respectively.