Senior police officer Saeed Khan Wazir says Tuesday's attack took place outside the regional office of the National Database and Registration Authority, which issues identity cards to Pakistanis.
He says the police are still trying to determine whether the attack was carried out by a suicide bomber or if the bomb was rigged to a motorcycle.
Wazir says the dead and wounded are being taken to hospitals.
No one immediately claimed responsibility for the attack but previous such attacks have been blamed on the Pakistani Taliban.
Mardan is about 50 kilometers (30 miles) northwest of Peshawar, the capital of Khyber Pakhtunkhwa province bordering Afghanistan.
Related story
At least 56 killed in suicide bombing at Shi'ite mosque in Paki...
