Mitchell played clips of Gregory's career highlights at NBC and "Meet the Press."
"In 20 years with NBC News, David has done it all," she said. "Through all the years, David has been true to the traditions of this program and NBC News."
She concluded by saying goodbye to him as a coworker.
"As David leaves NBC News for his next adventure, I will miss him as a daily colleague, but I know he will always be a friend," Mitchell said.
Watch the tribute via NBC News: