Ghulam Abbas was shot dead in the Korangi area of the financial capital late on Monday night.
He was a witness in the murder of Sabeen Mehmud who was killed after hosting a gathering to discuss violence in Balochistan.
Police say they are not sure if the two killings are linked but are increasing protection for other witnesses.
Reports say two men on a motorbike shot Mr Abbas dead outside his home, before escaping. Police say he suffered three bullet wounds.
"Ghulam Abbas used to work as Sabeen Mehmud's part-time driver," police official Rao Iqbal told Dawn newspaper.
"The witnesses insisted that the attackers knew him personally so we can't reach any conclusion with authority about the motive behind the killing."
Ms Mehmud had been driving her car with Ghulam Abbas sitting on the rear seat when she was attacked in April. Mr Abbas drove the activist and her injured mother to hospital after the shooting.
Police say Mr Abbas had already identified the activist's killers and had no further role in the trial.
Ms Mehmud had hosted a talk on allegations of torture in the province of Balochistan, where security forces are fighting Taliban militants, Baloch separatists and other groups, moments before she was shot dead.
Gunmen in the Pakistani city of Karachi have killed the driver of a prominent activist who was murdered in April.