During the interwar period, Meller, who spoke fluent English and French, was employed by the Társadalmi Múzeum, the social museum of Budapest, as well as a language instructor. In 1923, she served as a consultant on the executive meeting of the WILPF held in Dresden and in 1926, she served as a delegate to the Paris Peace Congress The following year, when Vilma Glücklich, who was the president of FE died, Meller became the recognized leader of the organization. From the 1920s to 1935, she participated in the international congresses of IWSA and WILPF, urging disarmament, lobbying for women's issues to be part of the League of Nations platform, and pressing for international legislation to address statelessness and lack of legal protections for women. On one of those trips in 1924, Meller came to the United States and participated in several speaking engagements for WILPF. In 1932, in protest of the execution of and , Meller wrote a pamphlet against using capital punishment for those accused of political crimes. When the Nazis annexed Austria in 1938, Meller's colleague, Rosika Schwimmer tried to persuade Carrie Chapman Catt to sign an affidavit allowing both Meller and Sarolta Steinberger to emigrate to the United States. Catt refused on the grounds of her age and the responsibility that such an affidavit would entail.