Leaders Hamilton Academical failed to keep up their blistering start to the new season, with the newly promoted side being held to a 0-0 draw at home by Kilmarnock.
And Inverness made the most of that slip-up, despite a late scare at Tulloch Caledonian Stadium.
John Hughes' side were made to work for the three points, with visitors St Johnstone putting in a disciplined defensive performance that took them into the break at 0-0.
But Marley Watkins provided the breakthrough with an hour gone as he slotted home after a fine one-two with Aaron Doran. Ryan Christie then doubled the lead five minutes later with a left-footed effort from inside the box.
The hosts might have hoped to cruise home as the final whistle approached, but Brian Graham's header ensured a nervy end to the game.
Elsewhere in the Scottish Premiership, Ross County's miserable start was extended as they were thrashed 3-0 by Aberdeen away from home to make it seven defeats from as many games.
Adam Rooney and David Goodwillie got on the scoresheet in the first half before Peter Pawlett netted a third in the second period in a comfortable win for the hosts, who move up to seventh.
