A witness said a crowd of people frantically tried to save Mr Mitchell but could not. Mr Mitchell had worked as a dive instructor on Catalina for about 10 years.
He was a recently naturalised United States citizen, held joint degrees in religious studies and sociology, and was an accomplished musician.
The 20m boat Mr Mitchell was on, a scuba diving tour boat called King Neptune, was destroyed in the storm.
The local City of Avalon council said Mr Mitchell was "a hero and beloved member of the city family, who gave the ultimate sacrifice as he valiantly served the community defending life and property".
"Tim was on duty as a harbour patrol assistant, working from a patrol boat, trying to protect residents, visitors and boats that were being battered by the fierce winds and waves that quickly and mercilessly descended upon Avalon."
The council said Mr Mitchell joined the local harbour authority in May last year, after moving to the US in 2000.
A local boat owner saw Mr Mitchell in the cockpit being "buffeted around" before tragedy struck.
Several boats in the area broke free from their moorings about 10.55pm local time.
"I did see him fall off the boat. And at that point he was pinned between the rocks and the boat," Edward Ureno said. Witness John Etheridge said the boat hit the beach with "a great deal of force" a couple of times and "he was just thrown overboard".
Mr Mitchell's former colleagues in the local dive industry said he was a hero.
"Tim courageously attempted to save the King Neptune as she parted four mooring lines and was swept to Step Beach," the Catalina Scuba Luv company wrote in a Facebook tribute.
"One of the Harbour Department's boats came perilously close to hitting rocks, so one of the patrolmen jumped from the vessel to get to safety," the Los Angeles County Sheriff's Department said.
"Two deputies attempted to rescue him from the water but were unsuccessful.
"The patrolman became lodged between the vessel and the rocks under the turbulent waves and could not be reached."
Mr Mitchell's body was recovered the next morning.
He is survived by his parents, by two brothers and Michelle Rodriguez, his girlfriend.
Another man, 53-year-old Bruce Ryder, also died in the storm.