The 27-year-old joined through the Addicks academy until he was 14. He later impressed at non-League side Corinthian, earning a move to Millwall in 2015.
McNamara enjoyed loan spells with several clubs before breaking into Millwall’s Championship side in 2021.
He went on to make 166 appearances for the Lions, scoring twice, before leaving following the expiry of his contract.
"I’m absolutely delighted to get it done. I started my career here when I was really young, so it holds a special place in my heart and I can’t wait to get going again,” McNamara told club media.
"I know how big of a club Charlton are. I’ve got a lot of family and friends who are Charlton fans - it’s a great community and I’m delighted to be a part of it.
"As soon as I met the manager, I knew it was the right place for me. The ambitions of the club and what he wants to build is very exciting and I want to be a part of that."