Miller's converted try and a Jimmy Gopperth penalty put Wasps 10-3 up at the break after Saints' Stephen Myler kicked the first points of the game.
Myler swapped penalties with Elliot Daly before Miller grabbed his second.
Daly's long-range penalty and Jamie Stevenson's try ensured Wasps moved to within two points of second place.
They are also now seven points clear of fourth-placed Leicester Tigers with just three games remaining.
With fifth-placed Saints facing Leicester in their next league outing on 16 April, Wasps are on the verge of making their own top-four Premiership spot secure and are well set to earn a home semi-final date.
The win against a below-par Northampton was not convincing until the latter stages, with a combination of some spirited defence, sloppy Saints handling and the distance kicking of Daly keeping them in front.
Miller's first-half try, following a slick move from a break from deep inside their own half, helped to establish a seven-point interval lead.
Despite not being at their best, a Saints side that lost England prop Kieran Brookes and replacement scrum-half Lee Dickson to head injuries kept probing and stayed within touching distance.
And it was not until Miller crossed for his second try on 66 minutes, running a brilliant line from George Smith's inviting pass, that the game was made safe.
Replacement scrum-half Stevenson's score in the final minute gave hope of a four-try bonus-point win, but the home side had to make do with four points.
Wasps director of rugby Dai Young: "We didn't play anywhere near our best and have played a lot better this season. We were edgy and snapped at things.
"Of course we're happy with the win, but it's tinged with frustration over the way we played.
"We realise that this performance will not be good enough against Exeter. There are lots of things for us to work on and get right."
Northampton director of rugby Jim Mallinder: "Lee and Kieran are up and about. We will have to go through the protocols and hopefully they won't have any symptoms.
"I can't remember a period when we have had so many concussions, but I guess that is what happens. We will have to battle on.
"Dylan (Hartley) is getting better. We are not sure when he will return, but hopefully we will get him doing some exercise this week."
Wasps: Miller; Wade, Daly, S Piutau, Halai; Gopperth, Hampson; Mullan (capt), Johnson, Cooper-Woolley, Launchbury, Davies, Young, Smith, Hughes.
Replacements: Festuccia, McIntyre, Cittadini, Myall, Haskell, Stevenson, Jackson, Tagicakibau.
Northampton Saints: Foden; K Pisi, G Pisi, Mallinder, North, Myler, Fotuali'i; Waller, Haywood, Brookes, Lawes, Day, Wood (capt), Harrison, Dickinson.
Replacements: Marshall, Ma'afu, Hill, Craig, Fisher, Dickson, Hanrahan, Collins.
Rob Miller scored two tries as Wasps beat Northampton to boost their top-four hopes while also sealing leaders Saracens' place in the play-offs.