Williams, who will end a six-month NBC-imposed suspension in August, appeared on the network's "Today" show and said he was sorry for the inaccuracies.
"I got it wrong," Williams said. "I own this and I own up to this."
NBC said on Thursday that Williams, 56, will not return to his prestigious anchor job but will join the network's cable channel MSNBC as anchor of breaking news and special reports in mid-August.
Before his suspension, "Nightly News" was the nation's most-watched evening newscast and Williams one of television's best-known journalists and frequent guest on talk shows because of his geniality and sense of humor.
Williams came under scrutiny in January because he had told increasingly harrowing stories about coming under fire during a helicopter ride while covering the Iraq War in 2003.
Williams, who took the "Nightly News" anchor chair in 2004, described his suspension as "torture" but "absolutely necessary."
"Looking back, it had to have been ego that made me think I had to be sharper, funnier, quicker than anybody else, put myself closer to the action," he said.
"I told the story correctly for years before I told it incorrectly. I was not trying to mislead people. That to me is a huge difference here."
Asked if he knew he was telling an untrue story on air about the helicopter, Williams said, "No. It came from a bad place. It came from a sloppy choice of words. I told stories that were not true over the years. Looking back, it is very clear I never intended to. It got mixed up. It got turned around in my mind."
Williams was suspended without pay for six months in February as the network reviewed 10 years of his work, as well as his public appearances. The investigation revealed Williams "made a number of inaccurate statements about his own role and experiences covering events in the field," NBC said.
"I would like to say what has happened in the past has been identified and torn apart by me and has been fixed, has been dealt with," Williams said, "and going forward there are going to be different rules of the road."