Sent into bat, Sri Lankan batsmen did not capitalise on the good starts before Mathews (50) and Lahiru Thirimanne (23 not out) added 58 for the fifth wicket stand.
Mathews hit five boundaries and a six off 34 balls before holing out in the last over.
Experienced opener Tillakaratne Dilshan managed just seven before he was the first wicket to fall in the second over.
Kusal Perera (15) and skipper Dinesh Chandimal (22) added 32 for the second wicket but off-spinner Saeed Ajmal put on the initial brakes with 2-35.
Pakistan gave an international debut to 19-year-old Usman Shinwari after the lef-arm paceman was drafted into the team as late as on Tuesday.
The second match will also be played in Dubai on Friday.