The dual South African-British national worked with handicapped Afghans and was killed in the western part of Kabul as she was walking to work around 8 a.m., officials said.
The gunmen, who were on a motorbike, shot the woman in the body and leg with a pistol, said Interior Ministry spokesman Zemeri Bashary.
The Taliban claimed responsibility for the slaying.
"This woman came to Afghanistan to teach Christianity to the people of Afghanistan," militant spokesman Zabiullah Mujahid told The Associated Press. "Our (leaders) issued a decree to kill this woman. This morning our people killed her in Kabul."
"She was a person who always loved the Afghans and was dedicated to serving those who are disabled," it said.
The group describes itself as a Christian charity registered in Britain. The Web site says it has been working with Afghan refugees since 1980 in Pakistan.
"SERVE Afghanistan's purpose is to express God's love and bring hope by serving the people of Afghanistan, especially the needy, as we seek to address personal, social and environmental needs," the site says.
Rina Vamberende, a spokeswoman for SERVE in Kabul, said the group is a Christian organization "but they are definitely not expressing this on purpose. They are here to do NGO (aid) work."
"It's not the case that they preach, not at all," she said.