[3] Andreu began work as a teacher in the model school affiliated with the university, but soon returned home to Fajardo where she continued teaching.

That same year, she became the vice president of the Puerto Rican Feminist League (Spanish: Liga Femínea Puertorriqueña), which was founded by Ana Roqué (1853-1933).

[1] The League was active in trying to gain women the right to vote and when their local senator, Antonio Rafael Barceló, refused to discuss the matter, Andreu, María L. de Ashford and Milagros Benet de Mewton went to plead their case in Washington, D.C.[4] In 1921, the organization changed its name to the Suffragist Social League (Spanish: La Liga Social Sufragista) and expanded its platform women's suffrage to full civic and political participation.

The following year Roqué and Andreu formed the Puerto Rican Association of Women Suffragists (Spanish: Asociación Puertorriqueña de Mujeres Sufragistas).

[3] Throughout the 1930s and 1940s, she continued her trusteeships with the Library and University, as well as public speaking engagements[11] and writing on topics ranging from education to women's rights to adult literacy.