The unconstitutional act that led to the U.S. Supreme Court's ruling in Brown v. Board of Education was the doctrine of separate but equal, established by Plessy v. Ferguson in 1896. This legal precedent upheld racial segregation in public education and other facilities. In 1954, the Supreme Court unanimously ruled that segregated schools were inherently unequal and therefore unconstitutional, reversing Plessy's decision. This landmark ruling was a crucial victory for the civil rights movement, paving the way for future legal challenges against racial discrimination and segregation across the United States.