McMillan became a teacher in 1927 in Caroline County, Maryland teaching at Denton High School.

During her presidency, the National Office was threatened with bankruptcy in 1976 due to legal proceedings against it in connection with a 1966 boycott of white merchants in Port Gibson, Mississippi.

She launched a fundraising drive to help defray expenses, and her efforts resulted in the Baltimore branch raising the largest local contribution of $150,000.

The role at the time was largely ceremonial, but McMillan had considerable influence on the organization's policies and operations.

[1] McMilllan was an outspoken critic of the Reagan Administration, which she felt harmed the NAACP's advocacy efforts in housing, education, employment and business.

[3] During her tenure, she also helped black businesses to receive federal contracts, and, in 1985, led a protest in Washington against South Africa's apartheid system.