He then participated in the Preparatory Committee for Indonesian Independence, lobbying other Muslim leaders to compromise in favor of national unity.

After the proclamation of Indonesian independence in August 1945, he served as chairman of KNIP for two months before joining Masyumi and being appointed attorney general in November.

After the end of the revolution, Kasman became a legislator and participated in the Constitutional Assembly of Indonesia in the 1950s where he endorsed Islam as the basis of the Indonesian state.

[1] During his time at STOVIA's preparatory school, Kasman was elected chairman of the Jong Islamieten Bond, a Muslim youth organization founded in 1925, and he held this position between 1930 and 1935.

[9] He was imprisoned by Dutch authorities for four months after he gave a public speech in May 1940 calling for Indonesian independence, but was released after a defense led by lawyer and fellow nationalist politician Sartono.

[14] After the takeover, Kasman was recruited into the occupational militia Pembela Tanah Air (PETA) and was appointed a battalion commander.

[20][21] The following day, the leaders formally requested the dissolution of PETA, although Kasman was appointed into a planning committee for an Indonesian national defence force.

[22] After PPKI's dissolution, the Central Indonesian National Committee (KNIP) was formed on 29 August 1945 with Kasman as chairman.

[25] Kasman was removed from his chairmanship on 16 October, with Adam Malik temporarily taking his seat until Sutan Sjahrir was elected the following day.

[23] In November 1945, following the formation of the Masyumi political party, Kasman was appointed as a deputy chairman in its inaugural leadership.

[12][28] During his brief tenure, he issued an announcement to regional leaders, attorneys and police officers calling for the implementation of the "rule of law through quick and fair trials".

[33] During the liberal democracy period in the 1950s, Kasman became a member of the Provisional People's Representative Council[8] and the Constitutional Assembly of Indonesia.

Prior Masyumi theorists had linked shura with democratic processes and ideals such as universal suffrage, while Kasman's interpretation limited it to a smaller group of Muslims – and excluded those which supported Pancasila.

[40] Shortly after the rebellion's outbreak, Kasman gave a speech in Magelang which resulted in his arrest by authorities and imprisonment starting on 5 September 1958.

[47] In the 1968 book Renungan dari Tahanan reminiscing of his time in prison, Kasman asserted his view of the superiority of Islamic principles over Pancasila.

[48] Kasman supported the foundation of the Indonesian Muslims' Party (Parmusi) in 1968, and was nominated for its first leadership election although he withdrew.

[53] Kasman died on the evening of 25 October 1982 after receiving nine months of treatment for prostate cancer at the Jakarta Islamic Hospital.