The fire was accidental, the fire department said Tuesday afternoon, and was caused by the failure of an extension cord used with a portable heater.
Fire officials said the blaze was discovered by chance late Monday when a department unit went for fuel. The firefighters saw smoke and went to the house, found it on fire and called for help. Authorities said there had been no 911 call.
The woman was identified as Juanita Brown, of Northeast.
She was rescued shortly before midnight from the second floor of the two-story duplex, according to the department.
She was the second person to die in a fire in the city this year, the department said.
Officials were in the neighborhood Tuesday afternoon giving out free smoke detectors and informing people on the dangers of fires.
The other fatal fire that happened Feb. 11 along Darrington Street in Southeast Washington was deemed accidental and caused by someone smoking in bed, firefighters said.
Lily Hearn-Coates, 65, died in that blaze.