Left-hander Grant faced Karim Darwish, the Egyptian favourite who had reached the semis without dropping a game.
And after taking the opening game, the world No4 from Cairo looked to be heading to his eighth win over the Londoner since 1999.
Grant upped his game, however, and maintained the upper hand over the next three games to record a notable 12-14, 11-8, 13-11, 11-9 upset in 80 minutes.
The 31-year-old former world No9, now ranked 12 places below Darwish, is celebrating his 29th appearance in a PSA World Tour final, where he will meet Mosaad.
The tall 24-year-old Egyptian, was also taken to four games in the other semi-final - where, much to the disappointment of the partisan crowd, he ended the unexpected run of local star Ong Beng Hee.
Unseeded Sarawak-born Beng Hee was unable to extend his KL run - which included the scalp of giant-killing compatriot Mohd Nafiizwan Adnan, the surprise conqueror of second seed Mohd Azlan Iskandar in the first round.
Mosaad battled for 52 minutes to see off the Malaysian 11-6, 11-7, 8-11, 11-7.
The win takes the world No13 from Cairo into his second successive PSA Tour final - and one win away from his second title of the month after lifting the inaugural HKFC Meco Invitational trophy in Hong Kong.
RESULTS: PSA International 50 CIMB Nicol David KL Open, Kuala Lumpur, Malaysia
 Adrian Grant (ENG) bt  Karim Darwish (EGY) 12-14, 11-8, 13-11, 11-9 (80m)
 Omar Mosaad (EGY) bt Ong Beng Hee (MAS) 11-6, 11-7, 8-11, 11-7 (52m)
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