In 1955, city buses throughout the South were racially segregated. Whites boarded the bus first and sat front to back, while black riders sat back to front. On crowded buses blacks were required to give up their seats to whites and move to the back of the bus. In many cases, black riders had to pay their fare to the driver, then exit the bus and reenter through the back door.

Rosa Parks worked as a seamstress at a department store in Montgomery, Alabama and rode the bus every day. On December 1, 1955, Parks was returning home from work on a crowded bus when the driver ordered her to give up her seat to a white man. When Parks refused, she was arrested for violating a city ordinance that enforced segregation on city buses. Her action came 100 days after the racially motivated murder of 14-year-old Emmett Till in Mississippi, and 6 months after the Supreme Court ruled in Brown v. Board of Education that segregated schools were unconstitutional. It reflected a growing resentment toward segregation and discrimination among African Americans, and the sense that the Supreme Court was now on their side.

Parks' refusal spawned a city-wide bus boycott by the black citizens of Montgomery. The Women's Political Council organized a one-day boycott of city buses. When the city didn't respond, the Montgomery Improvement Association, led by a charismatic young minister named Martin Luther King Jr., was formed to continue the boycott. NAACP leader E.D. Nixon, for whom Parks worked, used her arrest to advance the issue of segregated buses in court. Months before Parks' arrest, four other women were arrested for refusing to give up their seats on a bus. Their case, Browder v. Gayle, eventually went to the United States Supreme Court.

The Montgomery bus boycott continued for months. Angry whites bombed Nixon and King's homes. Since most bus riders were black, and the buses were virtually empty, the loss of revenue for the city was significant. Some white women gave their black employees rides to work, and carpools continued, but many African Americans walked to work day after day. The boycott drew national attention and triggered the desegregation of buses in other southern cities, but Montgomery city officials refused to desegregate the buses. In June of 1956, the state of Alabama outlawed the NAACP for its role in the boycott.

The boycott lasted for 381 days. It ended on December 21, 1956 after the Supreme Court ruled in Browder v. Gayle that segregated seating on city buses was unconstitutional.