Tansey’s solitary strike on the stroke of half-time was enough to secure victory for the visitors despite Ryan Christie’s dismissal – for a second bookable offence – five minutes into the second half.
Opportunities in the early stages, were few and far between with the closest either side came to creating even a half-chance occurring in the 14th minute when Saints midfielder Kenny McLean fired wide of Dean Brill’s left-hand post.
Both sides appeared to be focused on dampening the threat of the opposing strike-force – with seven yellow cards brandished in a first half that would not live long in the memory.
On the verge of half-time, however, referee Willie Collum adjudged Mark McAusland to have fouled Billy McKay in the penalty area and pointed to the spot.
Tansey stepped up and converted the penalty to give Inverness the lead going into the interval.
St Mirren, who have only scored once en route to losing all of their Scottish Premiership home games this season, were given a real lifeline to end their dismal home form when referee Collum sent Christie from the field for his mis-timed challenge in the 50th minute.
The hosts, however, were unable to capitalise on their advantage with Adam Brown and John McGinn, in the closing exchanges, missing the only notable chances their extra man could produce.
Inverness move up to third in the table ahead of next week’s visit of Dundee United, while St Mirren, who remain 10th, have a chance to claim their first home victory next week when Ross County visit Paisley.