Born on 7 August 1881, he was educated at the Durban High School and then graduated with a law degree from the University of the Cape of Good Hope in 1905, and was admitted as an advocate (the South African equivalent of a barrister) in 1913.

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.

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.

Jansen was a founder member of the South African Academy for Science and Art (Suid-Afrikaanse Akademie vir Wetenskap en Kuns - SAAWEK) in 1909, of the Co-operation Union (Saamwerk-Unie) in 1917, of the Federation of Afrikaner Cultural Associations (Federasie van Afrikaanse Kultuurvereniginge - FAK) in 1929, and of the Voortrekkers (the Afrikaner equivalent of the Boy Scouts and Girl Guides) in 1930, and was master of ceremonies at the laying of the foundation stone of the Voortrekker Monument in 1938 and at its dedication in 1949.