The club have confirmed the 29-year-old - who suffered a season-ending rib injury against Warrington Wolves two weeks ago - will leave when his contract expires this autumn.
The former Hull KR and Wakefield Trinity Wildcats back-rower joined Tigers in 2014, but been plagued by injury throughout the past two seasons.
Tigers coach Daryl Powell confirmed: “Frankie, unfortunately, has received an injury which will prevent him from playing the remainder of this season and we have decided not to extend his contract.
“He has been a great bloke to work with and has put in some awesome efforts for the team during his time at the club.
“I would like to wish him all the best for the future and thank him for his time at the Tigers.”
Mariano said: “I’d just like to say a big thank you to everyone at Castleford Tigers.
“I’d like to thank the coaching staff for developing me as a player, the physio team for looking after me during my injuries, the boys for being there on and off the field and the board of directors and staff behind the scenes.
“It really is a close-knit club. A massive thank you also goes out to the Tigers fans - their support has been amazing and something I will treasure and of course my biggest thanks go to my family who’ve been there for me and helped me through my injuries.
“I wish everyone at Castleford Tigers all the best for the future.”
Tigers chief executive Steve Gill described Mariano as “a great bloke to have around the club”.
He added: “He will be missed. He has given us some memorable moments, especially his try against Catalans this season that confirmed our Super League status and brought the house down.”
He is the third player known to be leaving Tigers at the end of the year, alongside full-back Luke Dorn and prop Lee Jewitt.