Simeón took on the position of Minister of the Environment in 1986 at a time when the issue was beginning to feature prominently in the political agenda of Governments and international organizations.
In 1989, she participated, for the first time, in the Forum of Ministers of the Environment of Latin America and the Caribbean, whose sixth meeting took place in Brasília that year.
Under Simeón's presidency, this event marked a significant change in those forums, with a shift of approach in the way the United Nations Environment Programme (UNEP) dealt with environmental problems in the region.
She was elected a member of the Bureau of the UNEP Governing Council in 2000, participating in the World Summit on Sustainable Development in South Africa in 2002.
In 2003, after her role in the negotiation of its headquarters agreement, she presided over the sixth meeting of the Conference of the Parties to the United Nations Convention to Combat Desertification, which took place in Havana.