He was born in Abeokuta, Ogun State, on 24 February 1917 to the family of an Egba policeman.

Rosiji attended Christ Church Primary School, Abeokuta, and then Ibadan Grammar School and Government College, Ibadan, for secondary education.

He was also educated at the Yaba Higher College, where he received a civil engineering certificate.

He subsequently went abroad to study for a law degree at the University of London in London, England after working at Shell Nigeria as an engineer.

Returning to Nigeria, he became one of the founding members of the Action Group.