John Hazel put the Capital side ahead in the first half, and then they went on to dominate the game after the interval.
Pat Stanton was back on duty for Hibs after his outing for Scotland in midweek. He replaced John Hamilton, and this was the only change from this side which beat Partick Thistle.
Stanton was in midfield alongside Edwards and Billy McEwan.
Stanton caused Saints a moment of worry with a dangerous ball which O'Rourke headed narrowly wide as Donaldson stood beaten.
The ball whizzed from end to end with neither side able to command the play. However, St Johnstone forced two corners and the pressure lifted when Whitelaw hit a right-footer twice. But in 29 minutes, Hibs took the lead. Edwards sent O'Rourke clear on the right, the centre drew Donaldson and tapped the ball inside for Hazel to sidefoot the ball into the net.
A sudden break out of defence gave Saints a chance. Aitken raced deep into Hibs territory and passed to Connolly who was brought down just outside the box, a situation that the Muirton men wasted.
Six minutes from half-time Donaldson punched aside cleverly. Hibs were on top. A corner followed and Hazel shot wide with panic showing in the Perth ranks. There was a brief stoppage while Black had attention, and then Stanton and O'Rourke teased the Saints defence until the ball ran loose.
Aird tried hard for St Johnstone, and he was foiled of a possible chance by Duncan's pace. However Aitken got in a header from the corner and once more Brownlie saved on the line. Hibs replied immediately and Duncan prodded the ball against the top of the bar. Donaldson denied Stanton a goal with a daring dive and the keeper was lucky to grab the ball when an Eric Schaedler shot deflected.
In 74 minutes, Edwards shot a second goal for Hibs from a Hazel pass.