Victoria Police was the lead agency in the search.
Victoria Police, SES, Ambulance Victoria, bush search and rescue and search rescue dogs were also on scene to support the search. Air, horse, bike and foot searches were conducted.
CFA crews from District 12 and 14 with support from other CFA members worked around the clock from Monday afternoon to find the missing teenager.
A lone bushman found William and alerted the authorities.
William ran off while walking with his dad and brother on a long weekend camping trip.
William, who has autism and is non-verbal, was last seen 1.5 kilometres south of Mt Disappointment.
CFA Commander Paul Elso said at least 50 members with 15 vehicles worked on each day shift and a reduced amount throughout the night.
“It’s been a great effort from all our member and it’s great to see such a positive outcome," Paul said.
“We’ve had an overwhelming amount of support from CFA members across Victoria. On request from Victoria Police, our forward planning was to provide crews for another four days around the clock.
“I’d like to thank everyone who has helped and offered their support. The search and rescue had nearly 500 people with many more members wanting to help."
CFA crews were able to assist the rescue by using thermal imaging cameras.
William is now back safe with his family.