Mendis ripped apart the UAE line-up plucking last five wickets in just 12 balls as the ACC qualifiers' innings was folded up for 148 in 36.3 overs, chasing a Lankan total of 290/9.
Opener Amjad Ali top-scored for UAE with 79-ball 77 laced with 12 boundaries.
Lanka rested the veteran trio -Sanath Jayasuriya, Muttiah Muralitharan and Chaminda Vaas - giving the youngsters a chance to prove their mettle. Mahela Udawatte and captain Mahela Jayawardene scored half centuries to help Islanders to a healthy 290/9.
The Emirate bowlers had restricted Sri Lanka to 209 for seven after 34 overs after Udawatte and Jayawardene had put on a second wicket total of 114 in 82 balls.
But some sensible batting coupled with quick running between the wickets from Kaushalya Weeraratne (40 off 54, 2X4s) and Nuwan Kulusekara (37 off 51) dampened an otherwise fine day for the UAE attack.
Seamer Zahid Shah was the pick of UAE bowlers with 3/49 while Fahad Alhashmi and offie Mohammad Tauqir took two wickets apiece.
Jayawardene, who made 61 from 43 balls with nine fours steadied the innings with a fine stand alongside Udawatte whose 67 included six fours.
After Jayawardene was run out, Alhashmi dismissed Udawatte and Tillakaratne Dilshan (24) while Tauqir got rid of Chamara Silva for six to slice through the middle order.
Weeraratne and Kulusekara then ensured Lanka post a solid total before Mendis ran amok to complete the rout.