The 2013 winner, a 9-1 shot under jockey Jamie Moore, hit the front with three fences to go to beat Firebird Flyer (25-1) and Shotgun Paddy (12-1) into second and third respectively.
Trained by Kerry Lee, the 11-year-old is the first double winner of the race since Bonanza Boy in 1989.
The race was postponed from 27 December because of a waterlogged track.
"He's not had a lot of racing," Lee told Channel 4 Racing. "He really enjoys the way he is trained. The thing is to keep him sweet, to keep him fresh and keep the variety going.
"I think the Grand National has to be his target now this season, he really enjoys soft ground and this kind of trip, so it has to be on the cards."
The race had been under threat after recent heavy rain, and Emperor's Choice, winner in 2014, pulled up with 10 fences to jump in heavy going.
Mountainous won by two and three-quarter lengths despite Firebird Flyer closing late on.
Limonali is the only other two-time winner of the race - and like Mountainous he won it twice in three runnings (1959 and 1961).
Coral Welsh Grand National result
1 Mountainous 9-1 (Kerry Lee) Jamie Moore
2 Firebird Flyer 25-1
3 Shotgun Paddy 12-1
4 Saroque 33-1
5 Woodford County 12-1
20 ran. Distances: 2¾ lengths, 11 lengths, 8 lengths
Mountainous won the Welsh National for the second time with an impressive triumph at Chepstow.