[1] Born in 1854, he was the second child and eldest son of Tēvita ʻUnga and Fifita Vavaʻu, the second daughter of Liufau, Tuʻi Haʻangana Ngata, and his second wife, Hulita Tuʻifua.

Although his paternal grandfather Tāufaʻāhau had become King George Tupou I of a united Tonga in 1845, Ngū's father was considered illegitimate by Christian standard because he was born to a secondary consort.

[2][3] After the death of his uncle Vuna Takitakimālohi, his father ʻUnga was legitimized and named Crown Prince under the terms of the first written constitution of Tonga on 4 November 1875.

While seeking medical attention in Auckland in 1879, his father died and Ngu succeeded as heir apparent to the Tongan throne, becoming the Crown Prince of Tonga on 18 December 1879.

[7] He left no legitimate issue so the position of heir apparent fell on his brother and then to their sister after his death.