An Australian teen YouTuber known mostly for his prank videos has been behind reuniting a family in the United States — almost by accident.

Brooke Roberts, who runs YouTube channel Prank Nation, posted a video of himself giving sandwiches to the homeless in downtown Los Angeles on July 4.

Shortly after, on the other side of the country in Florida, 10-year-old Evan Olsen was watching online videos for fun when he spotted his mum in Mr Roberts' video, sitting on a Skid Row street kerb and accepting a bag of food.

Jaime Garlinghouse had been missing for months after a long struggle with a mental disorder, and the family had feared she was dead.

Evan's father Eddie Olsen said the boy was overcome with emotion.

"Evan cried for an hour and a half, he couldn't even talk for 20 minutes. It was pretty amazing, he was so relieved to know that she's still alive," Mr Olsen said.

Eddie Olsen with son Evan. ( Supplied: Brooke Roberts )

He made contact with Mr Roberts in an attempt to track Ms Garlinghouse down. But when Mr Roberts returned to the place where he had first met her, she was nowhere to be found.

Two days of searching by Mr Roberts proved fruitless, so Mr Olsen made the cross-country trip to help find her.

Finally, after many more hours of searching, Mr Olsen was ready to give up. Then he walked down one final street.

"I started walking down the street that I wasn't going to go down for some reason, and all of a sudden Jaime came walking up and she couldn't believe that I was standing there," he told Mr Roberts over the phone moments after finding her.

"I feel like the universe guided me there because I didn't want to go that way and I wasn't going to go that way but I went that way anyway."

Ms Garlinghouse was able to go back to Mr Olsen's hotel room for a shower and to sleep safely in a bed for the first time in a long time. The next day, they went shopping for fresh clothes and other essentials.

Jaime Garlinghouse recovered in a hotel after months of living on the streets. ( Supplied: Brooke Roberts )

Bedtime phone calls from mum

It is not a story that ends with a tidy bow; Ms Garlinghouse is still living on the streets because, without official identification documents, she cannot travel by plane or even stay in homeless shelters.

But a crowdfunding campaign set up by Mr Olsen and Mr Roberts is helping her work towards a more stable life.

She also has a phone now, and talks to Evan before he goes to bed each night.

Mr Olsen said despite her mental health issues, the disappearance had been unusual.

"She was always a good mum, she was always there. When she disappeared in November and she missed [Evan's] birthday and she missed Christmas and she missed everything … I thought something bad happened to her," he said.

"Even with her mental state... she was still a good mum."

Mr Roberts, who hails from Adelaide and is only in the US for a month or two to film social media videos, has been in almost constant contact with Mr Olsen as the pair of them navigate Ms Garlinghouse's recovery.

The 19-year-old said he was still in shock.

"It's such a slim chance," he said.