In 1894, she became the first African-American woman licensed to practice law in Illinois, and the third in the United States.

[7][8] She worked in the Chicago office of Joseph Washington Errant, practicing probate and real estate law.

In 1896 she spoke at the national convention of the Colored Women's League in New York City, on "Woman in the Profession of Law".

[7] Ida Platt's cousin Richard Theodore Greener was the first African-American graduate of Harvard College, dean of Howard University's School of Law, and a diplomat in Siberia; his daughter Belle da Costa Greene was a prominent librarian.

[5][11] Today there is public housing for seniors in Chicago named the Ida Platt Apartments in her memory.