Scunthorpe could have also doubled their advantage, with Dagnall, again, going close when he headed Mark Duffy's free-kick narrowly wide of the top-right corner.
But the visitors were playing a full share in the game and almost levelled when Matthew Taylor's goalbound free-kick from long range was deflected wide for a corner.
Muntari had the first decent effort at goal, driving narrowly over with a 25-yard free-kick, while Herman Hreidarsson headed wide from a Kranjcar corner.
Pitman saw a free-kick deflected wide in the second minute, while the subsequent corner led to a quick succession of blocked headers and shots.
Maik Taylor tipped over Marek Jankulovski's wind assisted free-kick before Radek Sirl's inswinging corner was headed just wide from eight yards by Radoslav Kovac.
Substitute Merouane Zemmama's free-kick was deflected wide of the Motherwell goal and, from the resulting corner, Chris Hogg's header was claimed by Ruddy.
Black, until then the most influential player on the park, did not pack enough punch in his low spot kick and Stack stretched out a hand to turn the ball wide for a corner.
Mark McLaughlin headed a Marco Paixao corner just wide, then Saints' Graham Carey tested Tomas Cerny with a powerful late free-kick.
BBC: Alex Neil (r) and Steven Thomson in action at St Mirren Park
England goalkeeper Joe Hart has come under some criticism of late for his part in City's wobbles at the back but he was at his best to push Chris Brunt's free-kick wide and then tip Jonas Olsson's header over the bar from the resulting corner.
It only took 76-seconds for the Chiefs to make their attacking intentions known when Steenson's perfectly placed cross-field kick found Foster in space out wide, the winger duly taking advantage to cross in the corner.
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