The late penalty from Tim Holmer saw London Irish sneak past Northampton 14-13 at Franklin's Gardens on Saturday.
Toby Booth's men took an early lead through Homer's boot after just seven minutes.
But the majority of the first period was largely forgettable, Saints cancelling out that early strike with a Ryan Lamb penalty before the fly-half added a second against his old employers to make it 6-3 at the break.
Irish also found themselves reduced to 14 men on the stroke of half-time, prop Leo Halavatau being sent to the sin-bin by referee Martin Fox.
However below-par Saints were unable to capitalise on the one-man advantage and Homer levelled the scores with his second successful attempt.
With 10 minutes left on the clock Nick Kennedy finished off good work to score the opening try of the game, however the 6-11 lead was short-lived, with Saints being awarded a penalty try that Stephen Myler converted.
Dan Bowden's penalty kick to touch put Irish deep into enemy territory as the clock ticked down, and when referee Fox awarded a further penalty against Saints, Homer made no mistake with the all-important kick.
The irony of the dramatic finish will not be lost on Saints fans, nor Lamb, for it was his last-minute penalty in the first home game of the season at Franklin's Gardens that accounted for Gloucester, another of his former clubs.
Pens: Lamb 2
For London Irish:
Pens: Homer 3
Yellow card: Halavatau (London Irish - 40th min)
Northampton: 15 Jon Clarke, 14 Paul DIggin, 13 Tom May, 12 James Downey, 11 Noah Cato, 10 Ryan Lamb, 9 Martin Roberts, 8 Phil Dowson (c), 7 Ben Nutley, 6 Calum Clark, 5 Christian Day, 4 Samu Manoa, 3 Tom Mercey, 2 Brett Sharman, 1 Alex Waller.
Replacements: 16 Andy Long, 17 Paul Doran Jones, 18 Brian Mujati, 19 Mark Sorenson, 20 James Craig, 21 Stuart Commins, 22 Stephen Myler.
London Irish: 15 Tom Homer, 14 Topsy Ojo, 13 Jonathan Joseph, 12 Steve Shingler, 11 Adam Thompstone, 10 Dan Bowden, 9 Darren Allinson, 8. Chris Hala'ufia, 7 Alex Gray, 6 Ed Siggery, 5 Matt Garvey, 4 Nick Kennedy, 3 Leo Halavatau, 2 David Paice, 1 Clarke Dermody (c).
Replacements: 16 James Buckland, 17 Max Lahiff, 18 Mark George, 19 Bryn Evans, 20 Dave Sisi, 21 Guy Armitage, 22 Anthony Watson, 23 Paul Hodgson.
Venue: Franklin's Gardens
Referee: Martin Fox