The 23-year-old played 31 times last term but he featured for Grimsby and Eastleigh as well as his parent club AFC Wimbledon.
“I am pleased to bring Harry in because he was one of our first thoughts this summer,” Johnson told the club’s official website.
“I’ve always felt he hasn’t quite reached the potential I believe he can reach yet.
“He is a big lad, who is strong and a good athlete. He gets about the pitch and I think he can dictate games even more than he has been.
“I am really pleased because people I’ve spoken to enjoyed playing with him, they know of him and he is exactly what I need at this club at this moment in time.
“He is full of enthusiasm. He is relocating from his home in Essex, he’s a real Essex boy and could be in that show we all know and love!
“We both feel we can enhance each other’s future. He knows this season is very important to us and we need to get back into the Football League as quickly as we can.”