An ardent champion of Afrikaner interests, he joined the National Party in 1915 and was a member of Parliament from 1915 to 1920, from 1921 to 1943, and from 1947 to 1950.
In 1919, he was a member of a delegation which tried unsuccessfully to persuade American president Woodrow Wilson to call for independence to be restored to the former Boer republics of the Orange Free State and the Transvaal.
In Parliament, Jansen was Speaker of the House of Assembly of South Africa from 1924 to 1929, Minister of Native Affairs and of Irrigation from 1929 to 1934, and Speaker again from 1934 to 1943. He was highly regarded for his firm and impartial speakership.
He was Minister of Native Affairs again from 1948 to 1950, but was thought to be too soft on the new policy of apartheid, for which his department was primarily responsible. He was subsequently replaced by hardliner Hendrik Verwoerd and formally promoted by Prime Minister Daniel Malan to the politically neutral post of Governor-General once vacant. As an Afrikaner nationalist and stout republican, Jansen declined to wear the ceremonial uniform, or to take the oath of allegiance to the monarch whom he represented. He held office until his death in 1959, when he was succeeded by Minister of Justice Charles Robberts Swart.