The 26-year-old is out of contract at Stevenage at the end of June and will sign a two-year deal at Deepdale on July 1.
North End chairman Peter Ridsdale said: "John is an exciting, creative player who will be an excellent addition to the squad when he signs for us on July 1.
"He's our 11th summer signing and we're more than halfway through our recruitment process now. We've been working carefully towards getting out first choice targets in all areas and we're very happy with the progress we've made so far."
London-born Mousinho made 67 appearances for Stevenage, scoring 11 goals having joined in 2010 following spells at Brentford and Wycombe, but turned down the offer of a new deal to team back up with former Boro manager Westley.
Mousinho scored Stevenage's winning goal in the 2011 League Two play-off final at Old Trafford and finished that season as the club's joint top scorer.