The prominent figure who played a leading role in founding the American Civil Liberties Union (ACLU) was Roger Baldwin. Established in 1920, the ACLU was created to defend and preserve the individual rights and liberties guaranteed by the U.S. Constitution. Baldwin, a social activist and lawyer, was instrumental in pioneering efforts to safeguard civil liberties amidst growing concerns about oppression, particularly during times of war and political unrest. His commitment to civil rights, free speech, and social justice helped shape the organization's mission, making it a critical player in American legal history.