There is some controversy regarding the people around Martin  Luther  King Jr. at the time, but there's little out there to prove that  he was  a Communist, and polemics is not necessarily proof, and often isn't.
The FBI feared Levison was working as an "agent of  influence" over King,  in spite of its own reports in 1963 that Levison  had left the Party and  was no longer associated in business dealings  with them.
[197]  Another King lieutenant, Hunter Pitts O'Dell, was also linked to the  Communist Party by sworn testimony before the House Un-American  Activities Committee (HUAC).
[198]   However, by 1976 the FBI had acknowledged that it had not obtained any   evidence that King himself or the SCLC were actually involved with any   communist organizations.
[187]
http://en.wikipedia.org/wiki/Martin_luther_king
Don’t believe everything (or anything?) that Dyson writes. King — the   King of the Civil Rights Acts was certainly a centrist and even   conservative. We on the left — including SNCC and the Panthers and SDS —   regarded him with suspicion. We didn’t want integration into the   American system which we regarded as the Great Satan. Consequently, King   was pushed aside in 1966 and had no following. Under pressure from the   left he gave the worst speech of his career — the one about Vietnam —  in  order to regain his popularity among activists. The later Father  John  Neuhaus, then a radical, either wrote the speech or had an  enormous  influence on it. Martin Luther King’s message — America should  be a  society with a single standard for whites and blacks is anathema  to the  left and they have spent the last forty years subverting it.