While Dr. Martin Luther King, Jr. never held political office, his impact on American politics and public policy was huge.
Dr. King knowingly risked his life working for freedom and equality during dangerous times for blacks in America. He advocated using non-violent means, such as civil disobedience, to achieve change, and he was awarded a Nobel Peace Prize. Shortly before his assassination, he was an outspoken critic of the Vietnam War.
I do not think Dr. King was a libertarian. I think he had much more faith in the ability of government to fix unfairness and lift people out of poverty than most libertarians. However, I think most Libertarians agree with Dr. King's goals of ending government-imposed discrimination, segregation, and oppression.
The following is from the Preamble of the Libertarian Party Platform:
We believe that respect for individual rights is the essential precondition for a free and prosperous world, that force and fraud must be banished from human relationships, and that only through freedom can peace and prosperity be realized.
Consequently, we defend each person's right to engage in any activity that is peaceful and honest, and welcome the diversity that freedom brings. The world we seek to build is one where individuals are free to follow their own dreams in their own ways, without interference from government or any authoritarian power.
Libertarian National Committee
I would modify one sentence. To suggest that force and fraud be banished implies an impossible mission. We should do what we can to limit the use of force and fraud. We really have no other mission, do we?