The 27-year-old Kiwi should now remain at the club until the end of the 2016 season.
Niggling neck injuries have meant that Cahill has struggled to hold down a starting spot this year, and he has appeared in 12 games this season.
He is now fit, and hoping to boost the Chemics for the rest of their campaign, with a play-off spot now a real possibility.
“The contract is done and dusted and I can just focus on the rest of the season and I’m really excited about what’s to come,” said Cahill.
“I’ve had a few games now and I’m getting my match fitness back up. I’m only going to get better and hopefully I’ll move forward with the team and I can’t wait to rip in tonight and for the rest of the season.
“I’ve been here from the start of Super League and I’m part of something special which is being built here by Denis (Betts) at Widnes Vikings. I’m enjoying being here and playing for the Club and I think we’ve got a bright future ahead of us.
“The race for the top eight is hotting up now and we have a few teams right behind us, so it’s a good time to be involved. The boys are working hard but we know where we want to be and that’s play-off footy. We know what we have to do to get there and we will give it everything to get there.”