Pakistan now await the winners of the second semi-final between the West Indies and England to be played on Tuesday.
Mahmood and Alam pressed home the advantage given by their bowlers, who had restricted India to 169 in 47.3 overs.
Mahmood, who also took two wickets, scored an unbeaten 45 with a six and four boundaries, while Alam made a 50-ball 43 with seven fours after their team was struggling at 83 for five. Salman Qadir contributed 24 before falling to left-arm seamer Rudra Pratap Singh, who finished with 2-25.
Earlier, India struggled against some tight bowling and lost wickets at regular intervals after electing to bat.
Opener Shikhar Dhawan failed on the big occasion after slamming three centuries in the tournament as he was caught at mid-wicket trying to pull seamer Ali Imran. Praful Waghela departed in the next over as the Indians were reduced to 24 for two before opener Robin Uthappa (33) and Rakesh Solanki (21) added 49 for the third-wicket.
India missed the services of their skipper Ambati Rayudu, serving a one-match ban for slow over-rate in a super league match against Sri Lanka. Wicketkeeper Dinesh Karthik led the team.
Seamers Afridi and Ali Imran, and spinners Mahmood and Qadir bagged two wickets apiece.The final will be played on March 5.
India under-19 169 in 47.3 overs (Robin Uthappa 33, Sunny Singh 25, Rakesh Solanki 21, Abhishek Sharma 21 not out, Salman Qadir 2-27, Riaz Afridi 2-31, Tariq Mahmood 2-34, Ali Imran 2-38)
Pakistan under-19 171-5 in 44.5 overs (Tariq Mahmood 45 not out, Fawad Alam 43 not out, Salman Qadir 24, Rudra Pratap Singh 2-25).
In-form Indian colts raring to go against Pak