Her degree of Master of Laws (LLM), was obtained from Harvard University, in Cambridge, Massachusetts, United States.

[3] Mayanja joined the United Nations, soon after the First World Conference on Women, held in Mexico City from 19 June 1975 until 2 July 1975.

[4] She first worked in the Division for Equal Rights for Women in the United Nations' Centre for Social Development and Humanitarian Affairs.

Between 1995 until 1999, Mayanja held different senior positions in the Office of Human Resources Management, including as Chief, Common System and Specialist Service, dealing with policies regarding salaries and entitlements, as well as appeals and disciplinary cases.

In 2000, she was seconded by the United Nations to Food and Agriculture Organization (FAO) as the Director, Human Resources Management Division.