Two goals from Jon Obika either side of a Danny Mullen strike settled a scrappy contest as their East of Scotland League opponents defended heroically for long spells in front of their 1,600-strong travelling support.
St Mirren handed debuts to central defenders Conor McCarthy and Akin Famewo, who have arrived from Cork City and Norwich City respectively. Another new recruit, Jamie McGrath, started on the bench.
It was one-way traffic from the first whistle, with Obika blasting over the bar within the opening 35 seconds. Junior Morias then tested goalkeeper Connor Wallace with a low shot that was comfortably saved.
Saints should have been in front after 25 minutes. Morias fired in a ball across the six-yard box where Obika inexplicably dived in to slide it wide when Ilkay Durmus would have had a tap-in.
It was not until five minutes into the second half that Wallace was called into action again, tipping away Kyle Magennis' effort. Durmus then smashed a shot wide after being picked out by Morias.
St Mirren finally moved in front after 54 minutes. Calum Waters took a shot that seemed to be drifting wide when it bounced off Obika and landed in the net.
That meant Broxburn had to show a bit more ambition and Ross Nimmo had their first shot on target after an hour that Vaclav Hladky saved.
Saints had chances to add to their lead late on but Wallace made great saves from McGrath and Morias to keep the scoreline down.
Substitute Mullen finally made it 2-0 in the 89th minute, bending in a shot from the edge of the box, before crossing for Obika to bundle in the third shortly after.