Civil Rights Act of 1866 Law & Legal Definition. The Civil Rights Act of 1866 is a United States federal law that extended the rights of emancipated slaves.It was mainly intended to protect the civil rights of African-Americans, in the wake of the American Civil War.ivil Rights Act of 1866 Law & Legal Definition. The Civil Rights Act of 1866 is a United States federal law that extended the rights of emancipated slaves. It was mainly intended to protect the civil rights of African-Americans, in the wake of the American Civil War. The Act gave the right: