Mahoney knew early on that she wanted to become a nurse; possibly due to seeing immediate emergence of nurses during the American Civil War. Black women in the 19th century often had a difficult time becoming trained and licensed nurses. Nursing schools in the South rejected applications from African American women, whereas in the North, though the opportunity was still severely limited, African Americans had a greater chance at acceptance into training and graduate programs. As soon as the New England Hospital for Women and Children was created, she began to show an interest in nursing at age 18. The NEHWC became the first institution to offer a program allowing women to work towards entering the healthcare industry, which was predominantly led by men. She was admitted into a 16-month program at the New England Hospital for Women and Children (now the Dimock Community Health Center) at the age of 33, alongside 39 other students, in 1878. Her sister, Ellen Mahoney, also decided to attend the same nursing program but was unsuccessful in receiving her diploma. The criteria in which the hospital utilized while choosing students for their program emphasized that the 40 applicants would be "well and strong, between the ages of 21 and 31, and have a good reputation as to character and disposition". Out of a class of 40 students, she and two other white women were the only ones to receive their degree. It is presumed that the administration accepted Mahoney, despite not meeting the age criteria, because of her connection to the hospital through prior work as a cook, maid, and washerwoman there when she was 18 years old. Mahoney worked nearly 16 hours daily for the 15 years that she worked as a laborer.