Cran's older brother Edward said he did not know his brother was going to be on the show, which aired today.
"I'm still in a state of shock.
"Mel Gibson cut my brother's hair. I got the biggest shock of my life when I saw it on TV."
Edward said he called his brother straight after watching the show.
"He was in a slight state of disbelief. I think he is missing his hair a little bit."
Edward said he showed his family, including his parents and grandparents, the video at a family dinner tonight.
"Their jaws just hit the floor."
The Auckland University student was holidaying in Hollywood after completing a semester abroad at Pennsylvania State University.
"He was walking along the street and got scouted by someone on the show," Edward said.
This is not the first time William has sacrificed his luscious locks.
Edward said his brother shaved his head about five years ago for a friend who had cancer.
When asked by Gibson on the show how he would describe his hair, William replied; "poodle".
"You're gonna get sheared by Mel Gibson," Kimmel jokes after making a quip about the number of sheep in New Zealand.