In 1943, Leticia del Rosario was hired as an Assistant Professor in the Department of Physics of the University of Puerto Rico, Río Piedras campus. At the time, Jaime Benítez Rexach was the chancellor, physicist Facundo Bueso Sanllehí was dean of natural sciences, and Amador Cobas was chair of the physics department. After one year, she took a leave of absence to continue her doctoral studies. In 1948, after obtaining her PhD, she went back to UPR. Soon after that, in 1949, she was designated chair of the physics department. She held that position until 1954.