The 25-year-old, who was released by the Addicks at the end of the 2012/13 campaign, has penned a two-year contract with the option for a third.
Haynes agreed terms at Meadow Lane earlier in the week and the deal has now been completed.
County boss Chris Kiwomya is delighted to have secured the services of a proven goalscorer who was interesting a number of other sides.
He told the club's official website: "I think everyone was aware that one of our top priorities this summer was adding extra firepower to the squad and I knew very early on that Danny was the man that I wanted to bring in.
"It was far from straightforward, but then it never is when you're going after good players, they always have other clubs interested in them and that is reassuring in some ways.
"I know Danny well, having seen him play at the very start of his career and I think he's the perfect fit for Notts County, he's a player that has the ability to excite the fans and we're looking forward to seeing him in action."