[3] He settled in Melbourne and proclaimed himself a leader of the Cambodian expatriate committee in that city.
He moved back to Cambodia in 1991 as the communist government was falling, and became a high-ranking official in the FUNCINPEC party.
In July 1997, FUNCINPEC leader Norodom Ranariddh, who was serving as first prime minister, was deposed by second prime minister Hun Sen of FUNCINPEC's rival and coalition partner, the Cambodian People's Party.
[citation needed] Hun invited Ung to become first prime minister to replace Ranariddh.
Ranariddh's father, King Norodom Sihanouk at first refused to recognize the arrangement, but Ung became first prime minister in August 1997 after being elected by Parliament.