Tony Andreu's 17th-minute free-kick put the visitors ahead before McKay, who last scored against the same opponents on the opening day, drilled in an equaliser.
A 59th-minute header from Gary Warren and McKay's second handed Inverness breathing space but Andreu smuggled home his second to reduce the arrears.
James Vincent made sure of the points three minutes from time, lifting John Hughes' side above Celtic, Hamilton and Dundee United, who lost 1-0 at Motherwell on Friday night.
Champions Celtic have to chance to top the table on goal difference with a victory at Aberdeen on Sunday.
Bottom side Ross County condemned Kilmarnock to a third consecutive loss as three first-half goals secured a 3-0 triumph at Rugby Park.
Graham Carey opened the scoring with a shot that crashed against the underside of the crossbar after 33 minutes before Michael Gardyne slotted home and Paul Quinn netted a volley to complete a woeful spell before the interval for the home team.
Ross County are now level on points with St Mirren after the latter slipped to a 1-0 defeat against Partick Thistle, courtesy of a Christie Elliott goal 15 minutes from time
St Mirren's tally of one point from seven home matches was almost improved upon when Adam Drury hit the crossbar during the closing stages.
Elsewhere, Dundee and St Johnstone shared a 1-1 draw at Dens Park, where David Clarkson scored his sixth goal in as many games but Brian Graham's penalty ensured honours ended even.
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