The news follows the pairs' success with the Reds since taking charge in June 2019, including an unbeaten start to the 2020/21 campaign before the season came to a halt.
"We came here with a plan of where we want to go and we haven't finished that yet," said Grainger.
"Anybody who knows me knows that I'm ambitious and that I want to keep being successful.
"So, obviously, for me and Ruddy to be able to get our future's tied down and get it sorted, we're excited for what's going to lay in front of us."
Chairman Dave Bowden added: "We're absolutely delighted.
"It's obviously something important, in terms of looking forward, that we got Danny and Steven sorted out.
"We're very pleased about it."