Jordan Jones bagged an incredible first goal in the stripes as the Buddies eased into the Scottish Cup quarter finals.
Jim Goodwin's side's progression over Kelty Hearts never really looked in doubt as the game progressed, with Alex Greive proving his striking instinct once again as he netted the opener early from close range.
The tie was then taken away from the visitors at the start of the second half as Jones curled a superb strike into the top corner, opening his account for the Saints in stunning style.
Greg Kiltie bagged himself a brace to ensure a comfortable safe passage into the next round, with Kelty hardly laying a glove on the Paisley side who stretched their unbeaten run in 2022 to seven games.
Kelty Hearts were hoping to pull off a Saints double, having sent current holders St Johnstone out of the competition in the previous round.
Jim Goodwin spoke before the game that he wouldn't be taking the League Two side lightly, and he was as good as his word. He resisted the temptation to rest key players, naming the exact same starting XI that toppled the Perthshire Saints just three days before.
Kevin Thomson's side started the game fairly brightly in the Paisley sunshine. Joe Cardle theatrically bought the visitors a free-kick on the edge of the box on the five-minute mark. Skipper Michael Tidser slammed the chance wastefully over the bar.
St Mirren captain Joe Shaughnessy was then called into action, pulling off a crucial block as Joseph Cardle released a shot from close-range.
The Saints made Kelty pay by racing into an early lead. Marcus Fraser's teasing cross found Richard Tait at the back post.
His acrobatic effort was well-saved by Darren Jamieson, but the ball fell kindly to Greive who was lurking inside the six-yard box. The New Zealand international leapt above his marker to nod home his second goal in as many matches.
The first half failed to catch fire after the opener, with St Mirren happy to keep possession without risking too much going forward.
Greive tried his luck from the edge of the box but in the end it was a comfortable save for Jamieson this time around.
Connor Ronan fired well over the bar from distance on his left foot as Goodwin's side looked to add to their lead.
Charles Dunne produced a vital tackle on Joseph Cardle just before half-time to ensure the Buddies ahead at the break.
Goodwin shuffled his pack at the start of the second half, with Scott Tanser making a welcome return from injury alongside Ryan Flynn.
The changes appeared to have the desired effect, as just five minutes into the second half the Buddies doubled their lead.
A short corner found Jones, who cut inside and curled a beautiful finish into the side netting to open his account for the Saints.
Goodwin's men were in cruise control and soon two turned into three.
Jones set up his former Killie pal Kiltie on the edge of the area, who swivelled past his man before unleashing a powerful strike that flew past Jamieson and nearly burst the net.
The Saints were proving why they were the Premiership side, with Greive denied his brace after a strong save from Jamieson.
Alnwick was properly called into action for the first time on the 65-minute mark, beating Alfredo Agyeman's low strike away to keep his clean sheet intact.
There was another bonus for the Buddies as centre-back Conor McCarthy made his first appearance in the stripes since November after overcoming his ankle injury. He was handed the last 20 minutes to help build up his match fitness.
Botti Biabi went close to grabbing a consolation as he found space in the box before firing into the Kelty fans behind Alnwick's goal.
In truth the Buddies progression was never really in doubt, with Kiltie adding the cherry on top with five minutes to go as he latched onto a horrendous Kelty pass to slide into an empty net.
