The four-year-old boy was found Friday afternoon by a neighbour "just in enough time" and appeared to be all right, Toronto police Const. Jenniferjit Sidhu said.
The child was supposed to be at home, she said, and his mother has been charged with abandoning a child under 10.
The arrest came a day after three-year-old Elijah Marsh died after spending hours in the bitter cold wearing only a shirt, a diaper and boots.
The heartbreaking story is prompting an outpouring of generosity from across the country.
Elijah walked out of the building early Thursday and he was found about six hours later in the corner of a nearby backyard. He was taken to hospital and police announced his death several hours later.
The news of Elijah's death was a "gut-wrenching moment" for Justin Kozuch, 36, a Toronto man who has a three-year-old son and nine-year-old daughter.
Kozuch said the tragedy really hit home, prompting him to start a fundraising campaign on Tilt.com to help raise money for the boy's funeral.
"As a parent we go to the ends of the earth to protect our children," he said. "For me, it was absolutely heartbreaking."
As of Friday afternoon, more than $118,000 had already been raised, a response Kozuch calls "wonderful." Tilt.com is waiving all credit card fees for those who contribute.
Kozuch said he plans to meet with Elijah's family soon, although he doesn't yet know what he'll say to them when he offers them the money raised.
"I can't imagine what they're going through," he said.
On the pavement in front of the apartment building where Elijah was last seen alive, people left flowers and stuffed animals in memory of the little boy with the braided hair and happy smile. And someone left a winter coat and snow pants.
Some contributors shared their thoughts after making a donation.
"This broke my heart," said Jennifer Yearwood. "It's the least I could do for that sweet little boy, Elijah."
"The generosity of people in times of such great loss and tragedy is heartwarming," said Dawn MacDonald. "God bless Elijah. You are now with the angels dear little man."
Mike Colle, who represents the riding in the provincial legislature, said he was bringing food and coffee over to the boy's family Friday afternoon.
Colle said he has contacted the local school trustee and the wheels are in motion to have Elijah's nearby daycare renamed after the boy.
"We can grieve, we can help with the funeral, we can do all this, but at least let's try and make a bit of a positive thing out of here," Colle said.
Security camera video showed Elijah pushing his way through two doors and leaving his grandmother's apartment building at about 4 a.m. Thursday when temperatures had fallen to about -20 C.
Family members, who notified police of his disappearance at about 7:30 a.m., said they had put him to bed Wednesday night and woke up to find him gone and the front door open, police said.
Officers on horseback, foot and in a helicopter searched for him, and police asked neighbours to check their yards, hallways and stairwells for the missing youngster.
Police Chief Bill Blair called it "a tragic set of circumstances."
Meanwhile, police said another three-year-old boy was found wandering outside naked in the city's west end Friday afternoon as the frigid temperatures rose to -15 C. The boy is well and the mother has been located, police said.