Final answer:
Lyndon B. Johnson was the president who was finally able to pass federal civil rights legislation by signing the Civil Rights Act of 1964 into law.
Step-by-step explanation:
The president who was finally able to pass federal civil rights legislation was Lyndon B. Johnson. He signed the Civil Rights Act of 1964 into law, which prohibited discrimination in public accommodations, housing, and jobs based on race, religion, national origin, and gender. Johnson's political maneuvering and influence in Congress were key factors in getting the legislation passed.