The home side created the first of numerous excellent chances in the first half in the 12th minute after great play from last week's sinner, Jason Scotland. The tricky winger turned two defenders and looked to have taken a touch too many before sliding the ball to Martin Hardie. Unfortunately for the Saints, Hardie's shot was weak and failed to cause County keeper Craig Samson any difficulty in holding the effort.
In the 34th minute Steven Milne was set up perfectly by Scotland and he should have given Samson no chance from point-blank range, but the Highland keeper kept the young striker out.
The former Dundee hit-man was not going to be denied however, recovering from that horror miss to open the scoring three minutes later after Hardie back-heeled an Andy Lawrie free-kick in the box from a suspiciously offside looking position.
St. Johnstone pulled further ahead in the 39th minute again through Hardie as he made no mistake after a dreadful error from full-back Gary Irvine presented him with the opportunity to beat Samson for a second time.
County pulled a goal back in the 53rd minute as midfielder Kevin Mckinlay rose in a crowded box to steer home an Irvine cross.
A draw would have been very difficult for the home side to take, such was their domination, but Derek Adams came extremely close to equalising, but his header from Michael Gardyne's cross went straight at Bryn Halliwell.
St.Johnstone eased their worries in the 82nd minute after Scotland had cut a swathe through th Stags defence leaving Paul Sheerin with thesimplest of tap-ins for the third goal in a convincing victory.