The former Manchester United man has struggled with persistent knee problems at the Stadium of Light but his team-mate is delighted to have the "world-class" centre-back fully fit
The 33-year-old defender has endured two injury-plagued seasons with the Black Cats and has not featured in a competitive game since January 2012 due to a recurring knee problem.
But Brown has been involved in Sunderland's current pre-season campaign, scoring in the Barclays Asia Trophy win over Tottenham, and Johnson feels the centre-back has made a huge impact.
"We've got a world-class defender back," Johnson told BBC Newcastle.
"He's been the best new signing we've had. Even though he's not a new signing, it has felt that way.
"It shows the type of player he is. We've seen how composed and top class he is and if we keep him fit we'll definitely be a stronger team."
The former Manchester United man has been limited to just 22 starts since being brought to the Stadium of Light by former manager Steve Bruce in July 2011.
Sunderland boss Paolo Di Canio has overhauled his squad this summer but previously revealed that Brown is among the old guard he wants to keep at the club, after initial reservations about his fitness.