The Pirates of the Caribbean star will play computer genius Frank Devereaux, according to Zap2it.
A fellow hunter, Devereaux will assist Sam (Jared Padalecki) and Dean (Jensen Ackles) in the season's sixth episode.
Recurring guest star Kim Rhodes will also reprise her role as Sheriff Jody Mills in the instalment, currently titled 'Slash Fiction'.
McNally's past television credits include Doctor Who, Life on Mars and Spooks. He has also acted in such films as 2008's Valkyrie and John Cusack's upcoming The Raven.
Former Buffy stars James Marsters and Charisma Carpenter, and Firefly actress Jewel Staite are among the other actors confirmed to appear in the new season of Supernatural.
The show returns to The CW on Friday, September 23 at 9/8c.
> Supernatural Jim Beaver teases seventh season 'changes'
Watch Jensen Ackles discuss Supernatural with Digital Spy below: