Fittingly, the winning points came from the boot of Karle Hammond, a Saints player until the end of last season, who dusted off his occasional goal-kicking skills to convert Peter Gill's try 14 minutes from time.
However, the Broncos owed their victory in equally large measure to some magnificent defence after they had taken the lead for the fourth time in a pulsating match - and this time held it.
They had hit the front early when Shaun Edwards, starting his first match since Wembley, sent Mat Toshack over with a perfect pass.
Saints hit back through Paul Sculthorpe, but Hammond's beautifully disguised kick opened the way for Tulsen Tollet, whose two conversions kept London ahead at the break despite Anthony Sullivan's try for Saints.
If that was a fluctuating half, the second was almost unbearable for the supporters of the two sides. First, Anthony Stewart put Saints back in front, but then Edwards kept the ball alive for Robbie Beazley to send Greg Fleming over.
When Sonny Nickle barged in for a soft try from dummy-half, however, Saints looked likely to get away with it.
But then Gill, playing in pain every week because of a chronic back injury, stretched out to score and Hammond landed only his second goal of the season.
Although the St Helens coach, Ellery Hanley, contemplating a fourth defeat in five matches, criticised his team for not putting the Broncos under enough pressure, there was still an enormous amount of work to be done and it could all have been in vain if Chris Smith had hung on to a pass in injury-time.
However, the Bronco's 17, who by now included a number of walking wounded, deserved their page in the annals of the game in London.
"We are coming away with two points that the club has never achieved before," said Hammond. "Team spirit is turning into team belief."
St Helens: Atcheson; Smith, Stewart, Newlove, Sullivan; Long, Wellens; O'Neill, Cunningham, Nickle, Joynt, Perelini, Sculthorpe. Substitutes used: Edmondson, Davidson, Price, Barrow
London Broncos: Fleming; Peters, Ryan, Tollet, Offiah; Hammond, Edwards; Cram, Beazley, Seibold, Retchless, Toshack, Gill. Substitutes used: Spencer, Simpson, Callaway, Wynyard.
Referee: R Connolly (Wigan).
n Hull are three points adrift at the foot of Super League after losing 26-17 to Huddersfield at The Boulevard yesterday. Tries from Dave Boughton, Darren Simpson and Craig Weston in the final 13 minutes yesterday enabled Huddersfield to allay their own relegation fears.Reuse content