Arriving sat the crease after South Wilts had lost two cheap wickets, Bain hit three sixes and 25 boundaries in his career-best knock.
He shared in two century partnerships, the first with Tom Baines (43) and later Kevin Chidwarl, whose hit an explosive 68 which included a six and ten fours.
South Wilts amassed 326-9 off 45 overs before dismissing Old Basing for 158 to win by 168 runs.