McMillan, 102, whose father was born a slave, played a key role in moving the NAACP headquarters from New York to Baltimore, the Baltimore Sun said Thursday. She also led the Baltimore branch and Maryland state conference of the NAACP.
Kweisi Mfume, former president and chief executive of the NAACP and a family friend called McMillan a "pillar of the civil rights movement."
House Democratic Leader Nancy Pelosi, who grew up in Baltimore, said in a statement that she was proud to have worked with McMillan to desegregate the city.
"It was because of Mrs. McMillan -- her commitment, her toil, and indeed, her disruptive thinking -- that brought the NAACP headquarters to Baltimore," Pelosi said.
McMillan, the oldest of four children, was an educator for 42 years. She earned a bachelor's degree in education from Howard University and a master's degree from Columbia University.
McMillan, who died Tuesday, is survived by a son and four grandchildren.