Final answer:
Thurgood Marshall was the lead lawyer for the NAACP in the effort to overturn the 'separate but equal' doctrine established by Plessy v. Ferguson, achieving monumental success with the Supreme Court's Brown v. Board of Education decision in 1954.
Step-by-step explanation:
The lead lawyer for the NAACP in the landmark effort to overturn the Plessy v. Ferguson decision, which established the principle of "separate but equal," was Thurgood Marshall. He brilliantly crafted the legal strategy which culminated in the unanimous Brown v. Board of Education decision by the Supreme Court in 1954, declaring that separate educational facilities are inherently unequal and thus, unconstitutional.
Thurgood Marshall later went on to become a Supreme Court Justice himself, continuing his life's work of fighting for equality and civil rights from the bench. The decision in Brown v. Board of Education was a pivotal moment in American history and civil rights, setting the stage for further legal challenges to racial segregation and discrimination.