Chinese artist Lei Yixin said Thursday the quote was removed by deepening the grooves along the side of the sculpture.
"All is going well," Lei said through his son, who served as interpreter.
The memorial, which opened in August 2011, featured the quote, "I was a drum major for justice, peace, and righteousness," which is a paraphrased version of a line King delivered during a sermon at an Atlanta church.
What King actually said was, "If you want to say that I was a drum major, say that I was a drum major for justice, say that I was a drum major for peace. I was a drum major for righteousness. And all of the other shallow things will not matter."
The paraphrased version caused a stir. Some thought it made King sound egotistical.
"He had no arrogance at all," poet Maya Angelou told The Washington Post in 2011. "He had a humility that comes from deep inside. The 'if' clause that is left out is salient. Leaving it out changes the meaning completely."
Work is continues on the memorial, which is expected to reopen by the end of August for the 50th anniversary commemoration of King's "I Have a Dream" speech, CNN reported.
Bob Vogel, superintendent of the National Mall and Memorial Parks, said the work to remove the passage should cost between $700,000 and $800,000, which will be paid for by a fund established for repairs.