He was famously known as the pioneer of organised anti-imperialism and early Malay nationalism within Malaya, which eventually culminated with the Malayan independence from Britain.

He was also responsible for the social and economic welfare of the Malays by setting up the Rural Industrial Development Authority (RIDA).

As Onn Jaafar's family had close relations with the Johor palace, Sultan Ibrahim treated him as an adopted son.

According to biographer Ramlah Adam, one of the main reasons for him to enroll at MCKK was the need to improve his Malay language proficiency that had weakened considerably following his time in England.

[3] In 1941, following the Japanese occupation of Malaya, Onn was drafted into the administrative system and served as a food controller in Johor.

News of the rally reached Sultan Ibrahim on 22 February, who was then residing at Grosvenor House Hotel in London.

Sultan Ibrahim approached the colonial office and withdrew his support of the proposal, but this did not appease the political dissidents and Onn continued to organise more rallies in the other Malay states to muster further support for his calls against the Malayan Union, and formed the United Malays National Organisation (UMNO) in May.

However, the IMP failed to receive sufficient backing from Malayans, and eventually Onn left it to form the Parti Negara, which placed membership restrictions on non-Malays in an attempt to appeal to Malays.

[10] Neither party gained popular support against Tunku Abdul Rahman's new Alliance coalition and he was eventually eclipsed in Malayan political life.

Dato' Onn died at the age of 66, on 19 January 1962 at the Officers' Ward, General Hospital, Johor Bahru.