The Addicks agreed a fee earlier in the week believed to be around £500k including incentives for Leeds.
Walton was quite happy to stay at United but Charlton's offer was to good to refuse.
"I am very grateful to the club for giving me the chance to move into The Premiership with Charlton," Walton told Stellar Group.
"I had a year left of my contract and was aiming for a good season with Leeds, but when this offer came out of the blue I knew it was too good to turn down.
"Charlton are an established Premiership team with an excellent manager in Iain Dowie and I am really looking forward to getting started with pre-season."
"I'm disappointed to lose a good youngster in Simon, but I felt I couldn't deny him," Kevin Blackwell said to Sky Sports News.
"Simon wished to speak with them, so I granted him that request."