Rafah Butros Toma was most likely killed because of her religion, Agence France-Presse reported.
The gunmen used weapons equipped with silencers to shoot her as she slept. She was the latest victim in a string of attacks on Christians in Iraq. In Egypt, Coptic Christians were victims of a New Year's Day bombing at their church in Cairo.
Toma, who lived alone in central Baghdad, was one of 120 people at the Our Lady of Salvation church who were taken hostage by suicide-vest-wearing terrorists on Oct. 31.
Iraqi security forces confronted the militants, three of whom detonated their vests. After the raid, 53 people, including two priests and Toma's 27-year-old cousin, were left dead. The church is one of the largest Catholic houses of worship in the country.
"I am attached to this place," Butros Toma told CNN when she and 100 other people came to the church 40 days after the attack, a traditional period of mourning for some communities in the Middle East. "Every other day I come here. I feel like my soul is in this place with them."
Toma had not been to church for three years when she finally went on Oct. 31. She told CNN that her cousin had threatened to stop visiting her if she did not go.
Butros said she could not forget the last words she said to him: "I will see you and talk to you after Mass."
On Dec. 30, at least two Christians were killed and 16 others wounded in a wave of bomb attacks on Christian targets in Baghdad.
Two U.S. service members were also killed in central Iraq Sunday night during an operation, the first deadly attack in 2011 on U.S. forces in Iraq, according to a statement released by the U.S. military.