I thought it was named for Franklin Pierces' VP Wm Rufus King.
It was changed to honor MLK a few years ago.
http://www.metrokc.gov/exec/mlk/motion.htm
MOTION NO. 6461
A MOTION setting forth the historical basis for "renaming" King County after the Reverend Dr. Martin Luther King, Jr., instead of William Rufus DeVane King for whom King County is currently named.