There are probably not that many people who have made more of a contribution to society than King did. Of course, he did not make this contribution all by himself, but he did lead the Civil Rights Movement and was very important in achieving the things that the movement attained.
Before the Civil Rights Movement, it was perfectly legal to discriminate against people based on their race. This is something that is completely unAmerican, at least in my mind. Even so, it was legal and accepted by most people in the United States at the time. The movement that King led was able to undo this system. It was able to put us on the road to the much greater level of racial equality that we have today.
By helping to lead America to achieve much more in the way of racial justice than it ever had before, King contributed greatly to society.
Are you talking about Martin Luther King or Martin Luther King Jr.?