The jury verdict means she will spend at least 30 years, and possibly life, in prison, according to Assistant District Attorney Thomas Lipscomb.
Judge Jeffrey P. Minehart ordered Spellman to undergo a mental-health evaluation as part of a presentence investigation. Sentencing was set for April 26.
Defense attorney Harry R. Seay declined to comment on the verdict or possible appeal. Spellman's family angrily denounced the verdict and trial. Bruce Stafford Sr., a retired police officer who is Spellman's paternal grandfather, decried the trial's fairness and the defense's ability to present evidence.
Spellman, now 19, was on trial for two robberies committed near her Cedarbrook home on the afternoon of Aug. 18, 2010.
Authorities said her accomplice was Von Combs, now 17, a neighborhood boy who knew her brother. Combs became the chief witness against her.
The first crime was the gunpoint robbery of Shirley Phillips in the 7700 block of Rugby Street. About two hours later, authorities said, Spellman shot and killed George "Bud" Greaves, a Navy Seabee who served on Guam during the war.
A devotee of physical fitness, Greaves was doing yard work when Spellman and Combs approached him outside his house on Pickering Avenue at Phil-Ellena Street.
Spellman drew a gun and demanded money, but Greaves grabbed the weapon and began wrestling with her. One shot was fired, the bullet went into Greaves' chest, and he bled to death internally in moments.
Greaves never married and was "something of a loner" but had his own small circle of friends, according to his cousin Myrtle Ryan, 85, who attended the trial almost daily with her son and daughter.
Ryan's daughter, Sandy DiGiulio, called the verdict "gratifying," adding, "We're glad to get some relief and resolution after all this time. But it's all just so sad and unfortunate. It's sad about this young girl."
Based on descriptions from Phillips and several of Greaves' neighbors, police arrested Combs and Spellman within a day.
Combs was tried as a juvenile in 2011 and found guilty of second-degree murder. Now in a Western Pennsylvania juvenile facility, Combs testified for the prosecution that Spellman suggested the robberies. He said it was her gun and that she used it to threaten Phillips and kill Greaves.
Spellman's statement to police said Combs carried the .25-caliber pistol and shot Greaves.
In closing arguments, Seay urged the jury to acquit Spellman. He argued that detectives framed Spellman by making her statement corroborate the earlier confession from Combs.
Seay noted that Spellman - who did not testify - was working in a doughnut shop at the time of the two robberies.
"So all of a sudden she's a gun moll, she's a gangster, she's carrying a pistol around and she's going to shoot somebody. Please!" Seay told the jury.
But Lipscomb argued that details of Combs' statement implicating Spellman were corroborated by other witnesses.
"Real life is much messier, but that does not keep you from finding the truth," Lipscomb said.
Contact Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or follow @joeslobo on Twitter.