The midfielder was on target twice before half-time at the Caledonian Stadium where Gary Fraser and Conrad Balatoni added further goals after the break.
Yet Caley Thistle had missed a great opportunity to open the scoring inside the opening 10 minutes when Marley Watkins fired wide with only the ‘keeper to beat.
But within two minutes Stevenson had grabbed his first of the afternoon as he found the bottom left-hand corner of the net from the edge of the six-yard box after latching onto Christie Elliot’s cross.
His second, on 40, arrived after Kallum Higginbotham had laid the ball into his path and Stevenson smashed the ball towards the top left-hand corner of the net from inside the box.
And it became 3-0 four minutes after the interval with Fraser heading in off the underside of the bar from close range, as he met Stuart Bannigan’s cross.
Bannigan also provided the assist for Balatoni to increase the misery on second-placed Inverness in the 58th minute when the defender nodded home from just a few yards out following a corner.
Inverness had kicked off hoping to draw level on points with leaders Celtic – who play on Sunday – but instead tasted defeat on home turf for the first time in a Scottish Premiership game since April 1.