Buddies dished up a Boxing Day treat
when he was yellow carded for a foul on Stuart Armstong after 30 minutes.
And, five minutes later, a moment of magic from McGinn made it 2-0.
After getting the break of the ball, he showed great skill to beat Gavin Gunning before sending a low drive into the far corner from 10 yards.
However, with Saints fans still celebrating that goal, United pulled one back.
Armstrong played in Ciftci and, although Kello got a hand to his shot, the ball trickled into the net, despite the best efforts of Darren McGregor to clear the danger.
At the other end, Newton saw a shot at goal being deflected over for a corner as Saints pushed forward.
Goodwin was substituted just two minutes before the interval, with fellow Irishman David van Zanten taking his place.
And Saints had to make another change at half-time as the injured McLean was replaced by Danny Grainger.
The start of the second half was littered with fouls and McGinn saw yellow in the 55th minute for a challenge on Paton.
Kello then came to Saints’ rescue as he got a touch to a Gary MacKaySteven effort.
And the Slovakian goalie was in action again after 68 minutes as he blocked a shot from Ciftci.
United were left facing an uphill battle after 70 minutes when Watson was shown a straight red for going through the back of Kelly.
And it took Saints less than a minute to make the most of their numerical advantage as Naismith was in the right place at the right time to take a McGinn pass and sidefoot home to make it 3-1.
Late on, Robertson fired a volley just wide after a shot had cannoned off McGregor’s body in the box.
Saints then capped a fine win as Thompson connected with Grainger’s cross from the left to lash a shot into the net from just inside the penalty area.