De'Marquis Elkins, 17, is charged as an adult with first-degree murder, along with a 14-year-old who was not identified because he is a juvenile, Police Chief Tobe Green said.
Police announced the arrest Friday afternoon after combing school records and canvassing neighborhoods searching for the pair. The chief said the motive of the "horrendous act" was still under investigation and the weapon had not been found.
The mother, Sherry West, wept Friday while she told The Associated Press that she pleaded with the gunman and a younger accomplice who approached her Thursday morning while she walked near their home in coastal Brunswick.
"He asked me for money and I said I didn't have it," she said. "When you have a baby, you spend all your money on babies. They're expensive. And he kept asking and I just said 'I don't have it.' And he said, 'Do you want me to kill your baby?' And I said, 'No, don't kill my baby!'"
West said the gunman fired four shots, the first into the ground. West didn't see a shell casing ejected and she assumed the gun wasn't real.
Then he fired at her head and the bullet grazed her left ear - she has a small scab and bruising there. He fired again and shot her in the left leg above the knee. "I didn't know I was hurt."
"The boy proceeded to go around to the stroller and he shot my baby in the face," she said. "And then he just shoved me when I started screaming and he ran down London Street with the little boy."
The mother lives in a rented house in the city's Old Town historic district. Beverly Anderson's husband owns the property and she said West has lived there for six or seven years.
"We're just very sorry about what happened and very aghast that something could happen in our little neighborhood," Anderson said. "It's a quiet, safe little neighborhood."
She said West stayed home to care for her baby, who was often spotted in his mother's arms.
"The house has a front porch with a swing and we'd see him out on the swing with his mother," Anderson said. "He was a happy, cheerful baby."
It's not the mother's first loss of a child to violence. Sherry West said her 18-year-old son, Shaun Glassey, was killed in New Jersey in 2008. She still has a newspaper clipping from the time.
Glassey was killed with a steak knife in March 2008 during an attack involving several other teens on a dark street corner in Gloucester County, N.J., according to news reports from the time.
"He and some other boys were going to ambush a kid," Bernie Weisenfeld, a spokesman for the Gloucester County prosecutor's office, told the AP Friday.
Glassey was armed with a knife, but the 17-year-old target of the attack was able to get the knife away from him "and Glassey ended up on the wrong end of the knife," Weisenfeld recalled.
Prosecutors decided the 17-year-old would not be charged because they determined that he acted in self-defense.
Associated Press Writer Christina A. Cassidy in Atlanta and news researcher Monika Mathur in New York contributed to this report.