Ms. Manthrinayake is a Board Director. She also serves as the legal advisor for the National Institute of Social Development (NISD). Ms. Manthrinayake also serves as a subject matter expert for the Centre for the Study of Human Rights attached to the Faculty of Law, University of Colombo.

A lawyer by profession, Ms. Manthrinayake has served in the capacities of project manager, lecturer, legal advisor, and social worker both locally and internationally. She became the first ever woman president of the Young Men’s Christian Association (YMCA) in Moratuwa, Sri Lanka and president of the Y’s Men’s Club of Young Moratuwa, a service club of the YMCA. She has served on the board for the Asia Pacific Alliance of YMCAs, along with serving as vice president for the Sarvodaya Women’s Movement.

Ms. Manthrinayake began her legal career at the Sarvodaya Legal Aid Services Movement. A year later, as a law graduate, she began working as a legal officer for a non-profit organization, Lawyers for Human Rights and Development (LHRD), where she worked for 12 years. She has been a project manager for the Women’s Network for Political Transformation project, which was funded by the Royal Norwegian Embassy, in different districts. She has taught at the Sri Lanka Foundation Institute, the Special Task Force Service in Sri Lanka, the Police Training College, and the Police Academy. She has participated in research on human trafficking in Sri Lanka, 'A Study on the Law and the Enforcement of the Law Relating to Trafficking in Sri Lanka in the Year 2003.'

Ms. Manthrinayake has a Masters in NGO Studies from Sungkonghoe University, Seoul, Korea, and a bachelor’s in law from the University of Colombo. She also has a Certificate and a Diploma in Counselling.