On 17 November 1997, Johnny was appointed as St. Lucia's first female ambassador to the OAS and the United States. She immediately became chair of the Leo Rowe Fund, which provides loans for fellowships and scholarships, which she headed for the next eight years. Johnny was a part of the delegation sent by CARICOM to Haiti after the 2004 Haitian coup d'état to evaluate the situation, being noted for her "shrewd negotiating skills and suave diplomacy" in the crisis, helped restore calm. In 2005, she was elected as Chief Negotiator for the Caribbean with regard to matters concerning the Organization of American States (OAS) and was elected as President of the OAS Permanent Council beginning in 2006. In 2007, Johnny resigned as ambassador and Permanent Council president and accepted a post to chair the Trade, Tourism, and Competitiveness Department of OAS, which she continued to head until 2010. In 2011 she became the Deputy Director of the Summits of the Americas Secretariat serving in that capacity until work was completed on the 2012 Summit of the Americas.