In Tam was born in Prek Kak village in Stung Treng district, Kampong Cham Province, in eastern Cambodia.

After serving as an inspector of the provincial militia, he eventually reached the rank of Brevet-General and rose to the position of governor of Takeo Province.

The latter took action in October 1971 to strip the National Assembly of its legislative powers, citing a growing state of emergency in the Cambodian Civil War: this provoked a protest by Tam and 400 monks.

A United States National Security Council report judged Tam to be among the most experienced and politically mature of Cambodian politicians at the time, with a deserved reputation for incorruptibility and a modest lifestyle.

[5] He refounded the Cambodian Democratic Party, which failed to win seats in Parliament in the 1993 elections under auspice of United Nations.