Aberdeen's 1-0 win over Dundee on Friday night had temporarily reduced the gap but the Hoops showed no signs of any pressure as Mackay-Steven grabbed his first goal of the season to open the scoring in the ninth minute.
Steven MacLean scored a surprise equaliser for St Johnstone, without a goal since December 5, three minutes later, but Stuart Armstrong restored Celtic's advantage with a superb volley in the 43rd minute.
Mackay-Steven slotted home his second of the match in the 55th minute to seal the win which means Celtic will hold a six-point lead over Aberdeen ahead of their top of the table clash on February 3.
Celtic had made two changes from the 8-1 thrashing of Hamilton with Stefan Johansen replacing the injured Tom Rogic while Kieran Tierney was favoured ahead of Emilio Izaguirre.
The goals had poured in for Celtic on Tuesday night and the signs looked ominous for St Johnstone when Mackay-Steven fired Celtic in front.
Tom Scobbie took the sting out of a Leigh Griffiths strike but the ball fell to Mackay-Steven in the box and he took a touch before drilling a shot past Alan Mannus.
However, St Johnstone grabbed a surprise equaliser in the 12th minute. Rangers target Michael O'Halloran's pace caused the Celtic defence problems and his cross from the by-line picked out MacLean who tapped the ball past Craig Gordon from a couple of yards out.
Nir Bitton, scorer of a spectacular strike on Tuesday, then stung the palms of Mannus with a vicious long-range shot before the keeper had to stretch to keep out a shot on the turn from Callum McGregor.
However, Mannus could do nothing to keep out Armstrong's volley just before the break. Mackay-Steven fed Lustig down the left wing and his cut-back from the left was met by Armstrong who fired an unstoppable volley into the net from 12 yards out.
Celtic made it 3-1 in the 55th minute through Mackay-Steven. With St Johnstone appealing for a foul on captain Brian Easton, Johansen raced forward and threaded a pass to the Celtic winger who controlled the ball before sending an angled shot beyond Mannus.
The Hoops pushed forward in search of a fourth with McGregor firing over before Mannus saved efforts from Griffiths and Armstrong.
St Johnstone came close to reducing the deficit on the hour mark but Celtic keeper Gordon made a point-blank save to deny Liam Craig after O'Halloran used his pace to get behind the Celtic defence again and pick the St Johnstone substitute out in the six-yard box.
Elsewhere, Dundee United showed they have the stomach for a relegation battle as the basement club thrashed second bottom Kilmarnock 5-1 to reduce the gap at the bottom to 11 points.