Rosberg extended his points advantage over Hamilton to 29 points at the last round in Austria, but despite the growing distance between them, Mansell believes the new-for-2014 championship format - with double points available at the 19th and final race at Yas Marina - will ensure the title fight goes down to the wire.
Asked at Silverstone who he thought would come out on top, Mansell said: "I'm not going to do either driver an injustice by saying I think it'll be one of them or the other [that wins] because I think they're both doing a fabulous job, as are Mercedes.
"I think it's going to go down to the last race. I really do believe that the last race with double points will decide the championship and I think it'll be very exciting if it goes that way."
Despite having scored one more victory than Rosberg, Hamilton has retired from two of the eight races so far, whilst the German has scored points in every one. However, Mansell, who competed in 187 Grands Prix for Lotus, Williams, Ferrari and McLaren, insists Hamilton needn't be concerned about being more than a race win behind Rosberg in the standings.
"Lewis is a few points behind, but there are so many races to go that it's nothing really with the way the system works now. In years gone by it would probably have been a bit of a deficit, but I don't see it as a hiccup.
"In fact, it can be an advantage [being behind at this time of the year] because if Lewis starts to perform a little better than Nico now, the pressure will transfer from Lewis to Nico because he'll be defending the championship points lead."
Asked if he thought Hamilton was currently being out-thought by his team mate, Mansell replied: "I think that's doing Nico an injustice. Nico is a super-quick driver who's been underrated and has always been there. He comes from great stock with his dad Keke being a world champion, he's just quieter and more thoughtful and he's doing a fabulous job. Lewis isn't doing anything wrong - he just needs to concentrate, settle down and have a bit more lady luck."
Indeed, Mansell believes that reliability could ultimately be the deciding factor in the championship.
"There is a reliability challenge with the new regulations this year," he said. "Are their cars both going to finish every race until the end of the year? I think it will come down to what kind of reliability each of them has.
"[Look at] Sebastian Vettel; for four years he did nothing wrong - everything was absolutely perfect. Now look what has happened this year - where's lady luck gone, where's the reliability gone? In F1 you still need that little 'something' that you can't buy or get someone to create for the car.
"I think it's going to be fascinating. But don't underrate Nico - he's a class act and a great driver. Lewis has got a challenge, but I think he's up for it."