The talented paceman finished with 5-35 off nine incisive overs as Pakistan defended their small total of 109 by bundling India out for a meagre 71 in the day-night match.
Pakistan, who won the Cup in Bangladesh two years ago, became the first team to defend the title.
None of the top-order Indian batsmen could read Ali, who swung the ball in at a lively pace to trap in-form opener Cheteshwar Pujara leg-before and then rattled the stumps of Rohit Sharma and Mayank Tehlan in his first over.
Ali was brilliantly supported by left-arm seamer Jamshed Ahmed, who bagged two wickets in his first two overs to reduce India to 9-6 in just 3.2 overs.
India could never recover from the horror start despite a fighting 39-run stand for the eighth wicket between Piyush Chawla (25 not out) and Pinal Shah (16), the only two batsmen to reach double-figures in a dismal performance.
Fast bowler Akhtar Ayub claimed the last three Indian wickets to spark celebrations in the Pakistani camp.
India, unbeaten and untested in the tournament before this match, were let down by batsmen after their spinners Piyush Chawla and Ravi Jadeja had done a commendable job to restrict Pakistan.
Leg-spinner Chawla finished with 4-8 off 8.1 overs, including two wickets in his opening over, and left-arm spinner Jadeja took three wickets for 16 runs.
Pakistan kept losing wickets at regular intervals against the Indian spin duo. Middle-order batsman Rameez Raja was the top-scorer with 25 off 67 balls.
Nasir Jamshed (18), Ali Khan (10), Sarfraz Ahmed (15) and Anwar Ali (17) were the other batsmen to reach double-figures.
Scoreboard
Pakistan Under-19s (50 overs maximum)
Nasir Jamshed c Ahmed b Jadeja 18
Ali Khan c Shah b Yomahesh 10
Mohammad Ibrahim lbw b Jadeja 0
Rameez Raja lbw b Sharma 25
Ali Asad b Piyush Chawla 2
Riaz Kail b Piyush Chawla 0
Sarfraz Ahmed st Shah b Bipinbhai 15
Anwar Ali c Pujara b Jadeja 17
Imad Wasim st Shah b Piyush Chawla 1
Jamshed Ahmed not out 1
Akhtar Ayub b Piyush Chawla 1
EXTRAS (b 3, lb 4, w 7, nb 5) 19
TOTAL (all out, 41.1 overs, 173 mins) 109
FALL OF WKTS: 1-24, 2-25, 3-39, 4-49, 5-49, 6-78, 7-93, 8-103, 9-103, 10-109.
BOWLING: Yomahesh 8-0-32-1 (3nb, 1w),Dhiman 1-0-5-0 (4w), Ahmed 6-1-15-0, Jadeja 8-1-16-3 (1nb, 1w), Piyush Chawla 8.1-3-8-4 (1w), Sharma 6-0-17-1, Bipinbhai 4-1-9-1.
India Under-19s
G Dhiman b Jamshed Ahmed 0
CA Pujara lbw b Anwar Ali 0
R Sharma b Anwar Ali 4
Ravikant Shukla b Jamshed Ahmed 1
M Tehlan b Anwar Ali 0
D Das c & b Anwar Ali 0
RS Jadeja c Nasir Jamshed b Anwar Ali 6
Piyush Chawla not out 25
P Shah c Sarfraz Ahmed b Akhtar Ayub 16
V Yomahesh lbw b Akhtar Ayub 3
PM Bipinbhai b Akhtar Ayub 0
EXTRAS (b 1, lb 2, w 6, nb 7) 16
TOTAL (all out, 18.5 overs, 107 mins) 71
FALL OF WKTS: 1-0, 2-5, 3-8, 4-8, 5-8, 6-9, 7-23, 8-62, 9-71, 10-71.
BOWLING: Jamshed Ahmed 7-2-24-2 (1nb, 1w), Anwar Ali 9-0-35-5 (6nb, 4w), Akhtar Ayub 2.5-0-9-3 (1w).—Agencies