USA won the toss and batted first. But they were bowled out for a mere 145 in 48.1 overs, an ICC media release said.
Dangerman Steven Taylor was dismissed by off-spinner Sanjam Regmi early on, but Aditya Mishra (28), Sushil Nadkarni (39) and Orlando Baker (39) held the USA innings together.
Nepal fielded well and took wickets at regular intervals. Basanta Regmi's left-arm spin was particularly effective and he notched up a haul of five for 20 to easily be the tournament's highest wicket-taker with 21 for a cost of just 140 runs.
Nepal lost Anil Mandal early, but then the other centurion of the competition, Subash Khakurel and one-drop Gyanendra Malla combined well.
Malla's 50 came off 54 balls (6x4, 1x6) before he eventually got out for 66 with the score on 102 in the 20th over.
Thereafter Nepal captain Paras Khadka walked in and steered the side home hitting the winning boundary in front of 250 jubilant Nepal fans.
Nepal's coach Pubudu Dassanayake was full of praise for his team: "The team did just what was needed here and deserved to win. When I came to Nepal the goal was always to get this team up to a chance of qualifying for the ICC Cricket World Cup 2015 and this Division 4 win now means we're one step closer. There's a lot more hard work to do but I'm delighted with the progress so far."
In the third and fourth place playoff at Selangor Turf Club Singapore emerged easy winner over Denmark thanks to Chetan Suryawanshi's all-round show. He opened the bowling to take three wickets and then donned the wicket-keeper's gloves and made a stumping. Denmark was skittled out for 83 just after the half-way stage.
Suryawanshi then opened the innings and made a rapid 45 (29b, 7x4, 2x6) as his team won by five wickets with more than 33 overs to spare.
Singapore and Denmark remain in Division 4 for the next cycle of Pepsi ICC World Cricket League events.
Meanwhile, Malaysia secured their second win of the tournament, this time again over Tanzania to win the fifth-sixth place playoff at the Bayeumas Oval.
Scores in Brief
Final: At Kinrara Academy Oval
USA 145 all out, 48.1 overs (Nadkarni 39, Baker 39; B.Regmi 5-20)
Nepal 147 for two, 28 overs (Khakurel 40 not out, Malla 66)
Nepal won by eight wickets
Player of the Match: Basanta Regmi (Nepal)
Third-Fourth playoff: At Selangor Turf Club
Denmark 83 all out, 26 overs (Suryawanshi 3-23)
Singapore 87 for five, 16.2 overs (Suryawanshi 45; Bashir 5-11)
Singapore won by five wickets
Player of the Match: Chetan Suryawanshi (Singapore)
Fifth-sixth playoff: At Bayuemas Oval
Tanzania 187 all out, 49.4 overs (Patwa 67, Mapunda 30; Ahad 3-33)
Malaysia 188 for 5 off 45.5 overs
Malaysia won by five wickets
Player of the Match: Abhik Patwa (Tanzania)