[1] Sein Win was born in Danyingon, Moulmein (now Mawlamyine), then under British Raj Burma on 19 March 1919[citation needed] as the son of Thin Myaing and Pwe Kyu.

In 1938, he moved back from Tavoy to Moulmein to study at Shin Maha Buddhaghosa National School (now BEHS 9 Mawlamyine).

The Great General Strike of 1938, known as Htaung thoun ya byei Ayeidawbon (the "Revolution of 1300" named after the Burmese calendar year), saw him as one of the district committee members of student protesters at Kyaikthanan picketing the colonial government.

In that year, he passed matriculation exam from Shin Maha Buddhaghosa National School and in 1940 he attended Police Officer Training in Mandalay.

In 1943, he married Mya Shwe, daughter of Captain Ba Pe and Daw Mone.