[2] She was also the United Nations Secretary-General's Special Adviser for Timor-Leste (East Timor) during 2013 to 2015, working to support peace-building, state-building, and sustainable development.

Prior to her appointment to ESCAP, Heyzer was the first woman from the South[clarification needed] to head the United Nations Development Fund for Women (UNIFEM).

Consequently, UNIFEM increased its resources tenfold, strengthened its programmes, ground presence and team leadership, and successfully advocated to put issues affecting women high on the agenda of the whole United Nations system.

[29] In 2009, Heyzer led an unprecedented dialogue with Myanmar's leaders on development and poverty reduction, resulting in the Government of Myanmar requesting the formation of a development partnership that has allowed practitioners and eminent international scholars, such as the Nobel Prize economist Joseph Stiglitz, and local researchers to exchange experiences and ideas with government agencies and civil society.

After assuming the post on 13 December 2021 succeeding Christine Schraner Burgener,[33][34] Heyzer set out a multi-track strategy promoting regional and international unity to support the will of the people of Myanmar and their aspirations for a peaceful, democratic and inclusive future.

[36][37] In response to the interview, a joint statement rejecting "[Heyzer's] proposal that those defying the military must negotiate a power sharing as a solution to the current political, human rights and humanitarian crisis created by the terrorist military junta" was published on 2 February and signed by 247 regional, international, and Myanmar civil society organizations (CSOs), led by Progressive Voice, the Women's League of Burma, and the Union of Karenni State Youth.

"[39] In the months following the contentious interview, Heyzer held extensive engagements with representatives of Myanmar civil society organizations (CSOs) and local humanitarian networks.

[41] Deputy Spokesperson for the Secretary-General Farhan Haq reiterated Heyzer’s intention to visit Myanmar in the daily briefing on 27 April, but noted that no dates had been formalized.

[42] Permission for Heyzer to travel to Myanmar was requested in January 2022 at the latest[43] and she received general approval by April[44] but was not granted the necessary authorizations for her stay nor for the people she could meet – which the United Nations has continued to insist must include various parties and not just representatives of the State Administration Council (SAC).

She further called upon the international community at large, and the nations of ASEAN in specific, to "actively engage women to fully address the humanitarian and protection needs of the people [of Myanmar] and amplify their voices for a future federal democratic union of peace, stability and shared prosperity.

[49] Regarding Heyzer’s exclusion, the Malaysian Minister of Foreign Affairs Saifuddin Abdullah tweeted, "We should not allow [the junta to be] dictating who to be invited for related meetings.

In her address to the session, she spotlighted the "urgency for a coherent international response built on regional unity" and insisted that more needed to be done to support local networks and to build community-based resilience.

She was on the High-Level Commonwealth Commission on Respect and Understanding chaired by Nobel Laureate Prof. Amartya Sen.[60][61] She was a jury member of US Secretary of State Hillary Clinton's Innovation Award for Women's and Girl's Empowerment, 2010.