Mohammad Zaki Abdu —the third secretary at the Afghan Consulate in Karachi — died of his wounds shortly after the shooting, according to the consulate's spokesman, Haris Khan.
"We were working at our office when we heard gunshots," he said. "Everybody was running in panic."
The guard, identified only as an Afghan national named Rahatullah, was taken into custody, said police official Azad Khan.
Both officials said the motive behind the killing was not yet known. The Afghan Embassy in Islamabad said it was awaiting further details, AP reported.
In Pakistan, which has long been a hotbed of militancy, foreign missions are provided extra security and also frequently hire their own private guards.
Rahatullah was the slain diplomat's personal bodyguard, and it wasn't clear whether the Afghan Consulate had hired him officially, said Khan, the police official.
Noor Wali Khan Noor, an official with the Foreign Ministry in Kabul, said he had no further details at the moment but that a delegation from the Afghan Embassy in Islamabad had been dispatched to Karachi to investigate the incident.