According to Đại Nam chính biên liệt truyện, he was born to the Empress Consort Phạm Thị Liên.
In 1790, Toản received the title An Nam quốc vương thế tử ("Crown Prince of Annam") from China.
Bùi Đắc Tuyên was favoured by the young emperor, and became the de facto ruler of the country.
Tuyên banished one of important ministers, Trần Văn Kỷ (陳文紀), from the capital Phú Xuân (mordern Huế).
Nhạc's eldest son, Nguyễn Văn Bảo, was granted the title Hiếu công by Toản.
Bùi Đắc Trụ (裴得宙) and Nguyễn Văn Huấn (阮文訓) were left in Quy Nhơn to watch Bảo.
Two Tây Sơn generals, Trần Viết Kết (陳曰結) and Nguyễn Văn Huấn, laid siege to Diên Khánh.
Several months later, Trần Quang Diệu and Nguyễn Văn Tứ (阮文賜) were sent to attack Diên Khánh again.
The power struggle destabilized the regime, which provided Nguyễn lord an opportunity to launch an offensive attack in 1797.
However, many ministers were accused of getting involved in the incident, including Lê Trung (黎忠) and Nguyễn Văn Huấn.
Trần Quang Diệu and Vũ Văn Dũng was sent to reinforce, but was ambushed by Nguyễn army in Thạch Tân.
Hearing the news of defeat, governor Lê Văn Thanh (黎文清) surrendered to Nguyễn Ánh.
However, Võ Tánh, the governor of Bình Định, suggested that he would pin the main Tây Sơn force down there so that Nguyễn Ánh could attack their capital Phú Xuân.
To make matters worse, Vientiane and Muang Phuan attacked Nghệ An Province, cooperating with Nguyễn forces.
Thiếp said it was impossible to deal with current affairs; however, if the capital could be relocated in Phượng Hoàng trung đô (鳳凰中都, in present-day Vinh), the lifetime of the dynasty might be prolonged.
In Nguyễn lord side, several Western adventurers joined the battle, including Jean-Baptiste Chaigneau, Philippe Vannier and Laurent André Barisy.
Nguyễn Quang Toản fled to Đồng Hới, with a dozen men, then to Thăng Long.
Meanwhile, Nguyễn Ánh also sent an envoy, extraditing Mo Guanfu, Liang Wengeng and Fan Wencai to China.
[3] In 1802, Nguyễn Quang Thùy was sent to attack Lũy Thầy (in present-day Quảng Bình Province).