Warriors general manager of football Dean Bell said Webster's credentials were impressive.
"Andrew is well regarded as a young coach on the way up," Bell said. "Cappy [McFadden] was keen to have a balance of an experienced voice and a younger coach as his assistants. Tony and Andrew will complement each other perfectly."
Webster said the offer to join the Warriors had come at the ideal time.
"I'm hugely excited about the opportunity to coach in the NRL and to have that chance with a club like the Warriors," Webster said. "I'm really impressed with the club overall, with the system and the quality of the players they always have coming through. The 10 years I've had in coaching has given me experience in every role possible in my goal to coach in the NRL. The chance to work with Cappy and Tony couldn't have come at a better time."
McFadden said he had followed Webster's development as a coach.
"I first became aware of him when I was coaching at Les Catalans in France. He was with Hull Kingston Rovers then and I've kept an eye on him since," he said.
"For such a young coach he has packed in lots of experience in Australia, England and the United States.
"I couldn't be happier with the calibre of the assistant coaches we've been able to secure for the next two seasons. Andrew and Tony are going to be invaluable with the support they'll give me and the squad generally."