Starting from 2012, she has been working as Deputy Director of the Ecosystems Division at the United Nations Environment Programme (UNEP). While doing so, she oversaw the organization's coordination, operations and programme delivery. She then became the Director of the Law Division at the United Nations Environment Programme (UNEP) in June 2014. In 2018, she again worked as the Acting Director of the "Corporate Services Division". In November 2019, Mrema worked as a temporary "Officer in Charge of the CBD Secretariat". Starting from December 2019, she worked as the Acting Executive Secretary of the United Nations Convention on Biological Diversity (CBD) Secretariat. In July 2020, she became the Executive Secretary role.