Firefighters, already on the scene, helped her into a ladder to safety.
The blaze started with the child playing with the stove in a first-floor apartment and then raced up the five-story building, causing what de Blasio earlier said was "one of the worst losses of life in many, many years" from a fire New York City.
The blaze broke out on Thursday evening in a 25-apartment building near the Bronx Zoo, one of the most popular tourist attractions in the United States financial capital.
Except for the September 11 terror attacks, it was the city's deadliest fire since 87 people were killed at a social club fire in the Bronx in 1990.
According to reports, Mensah's body was found in another apartment on the same floor, likely because he was trying to save others from the fire.
The fire started when a three-year-old child was playing with the burners on a stove top.
Young lived in the basement but had gone upstairs to visit Stewart-Francis in her fifth-floor apartment, said Young's boyfriend, Kenyon George.
The apartments stairwell acted as "a chimney" when the fire broke out into the building, according to FDNY Commissioner Daniel Nigro.
The city housing department said investigators would look into why the door did not close automatically, though Mayor Bill de Blasio said there was "nothing problematic about the building that contributed to this tragedy".
Five of the victims were pronounced dead at the scene, police say: an 8-month-old girl, a 63-year-old woman, and three unidentified men.
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At least 20 people scrambled out via fire escapes on a bitterly cold night, but others could not.
When the mother fled the burning apartment with the boy and his 2-year-old sibling, she made a fatal mistake - she left the apartment door open. Five of the building's residents were already dead when the fire department arrived three minutes after the first 911 call.
Many fled into the frigid night with just the clothes on their backs, and authorities called for those who wanted to donate clothes to do so at a local church.
"He just came for the holidays", he said of Emmanuel, who was staying in an apartment with friends.
"I still have the images in my mind. I'm still in shock", he said.
The building - cordoned off with yellow security tape - "is a memorial site now", he said.
Nigro said investigators had not yet determined how numerous building's fire alarms were working at the time of the fire. He became one of the 12 people who lost their lives in the inferno that took place in the Belmont section of the Bronx.
Nigro told reporters that investigators were still determining the condition of smoke detectors at the scene. Fire Department video released Friday showed the building's charred halls and stairs, where icicles had formed from water poured on the blaze. "They were burned, even little kids on the stretchers, burned", a resident of the building told ABC station WABC in New York City. A mother and three children were killed when a fire tore through their Brooklyn home on December 18.
The youngest victim died in the arms of her mother, who tried to escape the fire by hiding in the bathtub, the New York Post reported, citing law-enforcement and FDNY sources.
Mayor de Blasio told CNN on Friday, "This is a bad tragedy in the middle of the holidays".