Mellor, 25, joined Widnes on a dual registration contract with Wigan in 2011 before moving permanently in 2013.
"For him to commit to continuing his development with us in pursuit of realising his own ambitions is an honour," head coach Denis Betts said.
"He knows the way that we want to play and he has the attitude and energy that will keep enhancing the team."
Widnes Vikings scrum-half Joe Mellor has signed a new three-year contract, keeping him with the club until 2019.