In 1990, former President César Gaviria Trujillo named her as the head of the Presidential Advisory Office for the City of Medellín, where she gained national recognition for her social work in the most violent territories under the control of the drug cartel.

[6] Additionally, Mejía Velez championed women's rights by creating the Group of Friends for Gender Parity and United Nations which as of March 2020 was integrated by 149 Member States.

Mejía Vélez was appointed by former President Juan Manuel Santos as Ambassador Extraordinary and Plenipotentiary and Permanent Representative of Colombia to the United Nations in February 2014.

She finalized her term on October 1, 2018[7] During the course of her more than four years in the diplomatic post, she was involved with the adoption of the Sustainable Development Goals, the Addis Ababa Action Agenda of the Third International Conference on Financing for Development (Addis Ababa Action Agenda),[8] the reform of the selection and appointment process criteria of the United Nations Secretary-General, the establishment of a first Special Political Mission in Colombia and its Verification Mission,[9] as a result of the peace agreement between the Revolutionary Armed Forces of Colombia, among many other achievements in a context of a cycle of reforms of the organization.

"[10] Adopted by consensus on 11 September 2015, operatives paragraphs 32 and 38 modified the seventy-years old language to explicitly call for "Member States to consider presenting women as candidates for the position of Secretary-General.

The group was mindful of the strategy and commitments made towards achieving gender parity, particularly in senior positions, laid out in the vision statement and during the informal dialogues by the Secretary-General Guterres.