Mullally, 20, is a product of the Widnes academy system and moved Down Under last February to play for the Broncos’ Under 20 team in the National Youth Competition under the tutelate of head coach Matthew Wood.
The 6 foot 6 forward said: “As a young lad it was always my aim to play for the Vikings in Super League and with a bit of luck and a lot of hard work, hopefully I’ll achieve my goal.
“The nine and a half months I spent at the Broncos was rewarding in so many ways and I genuinely believe that I am a better player and person for the experience. Hopefully I can take the lessons I’ve learned into both pre-season and 2012.
“Obviously my aim, initially, is to break into the team and if I get my chance I know for certain that I’ll be giving it my all in an attempt to keep my place and get as many games as I can under my belt.”
Mullally returned from his overseas experience “a much fitter, faster and hungrier athlete” according to former Australian international Allan Langer, who is now employed as a skills coach at Brisbane Broncos.
The prop has been handed the number 26 in the Widnes squad list, while the Vikings are also expected to announce the signing of two more players to complete their Super League squad.