The victory means Mason is one shy of triple New Zealand rally champion Neil Allport's record of 22 round victories.
Sunday's six stages were won equally by Mason and West, but West's luck was again tested when he spent nearly one-and-a-half minutes stopped in the penultimate Berwick stage.
He managed to hold on to second ahead of Gilmour, while fourth overall was Christchurch's Matt Jansen and Jason Farmer.
Results from this weekend's Rally of Otago:
Richard Mason/Sara Mason, 2:37:47.2, 1; Chris West/Chris Cobham +2:13.7, 2; Emma Gilmour/Anthony McLoughlin +2:38.5, 3; Matt Jansen/Jason Farmer +6:33.8, 4; Dean Bond/Ross Moody +12:30.1, 5.
New Zealand Rally Championship drivers' championship points after round three:
Richard Mason 90, 1; Emma Gilmour 70, 2; Alex Kelsey 51, 3; Matt Jansen 48, 4; Chris West 44, 5.