Ainscough, 23, turned out for Wigan and Bradford in the top flight, before spending a year Down Under with South City Bulls.
Former Warrington and Castleford back-rower Mitchell, who also had a loan spell at Harlequins, was another Englishman to spend last year Down Under, but returns to the UK to link up with Dave Woods’ side.
Woods has been busy in the market, signing young full-back Tom Carr from York, prop Samir Tarhaoui from Swinton and stand-off Chris Taylor from Wigan.
He said: “These are great additions to our squad. The mixture of local youngsters and experienced SL players is ideal and players of this quality are hard to find.
“All credit to the Board of Directors and particularly Chief Executive Barry Richardson for their hard work in recruitment. We intend to aim for the play-offs next season and there will be more additions to the squad.”
Half-back or hooker Neil Thorman has also re-signed for another season.