Cobb, 29, of Rusk, in East Texas was executed with a lethal injection at 6 p.m. Thursday at the state prison in Huntsville, KTRE-TV, Lufkin, reported. Beunka Adams, who was a year older than Cobb, was put to death for the crime last year.
The U.S. Supreme Court refused to delay Cobb's execution, the fourth this year in Texas.
Elmer Beckworth, who was Cherokee County district attorney when Adams and Cobb were tried, said the "system is working."
"I feel for the victims in having to wait so long for closure, but the jury made the correct decision," Beckworth said. "The evidence clearly showed they were guilty of capital murder. The evidence showed they were both of future danger to society."
Adams and Cobb were arrested a few hours after two masked men robbed a convenience store in Rusk. The robbers kidnapped two female clerks and Kenneth Vandever, 37, of Rusk, drove them to a pea patch and sexually assaulted one of the women.
All three were shot but the two women survived. Beckworth said Vandever, who had suffered a brain injury in a car crash, liked to hang out at the convenience store to socialize, KWTX-TV, Waco, reported.
Adams and Cobb met in a boot camp several years before the robbery. Investigators said they became partners in crime, first committing burglaries and then moving on to armed robberies.