Tom May and Luther Burrell both scored a brace of tries to send Welsh back down to the Championship and seal a vital bonus point victory as the Saints look to secure a place in the top four.
The struggling hosts started promisingly and were ahead after half an hour as winger Nick Scott scored and scrum-half Alex Davies added the conversion in response to two penalties from Stephen Myler.
The Exiles' challenge unravelled somewhat in the final 10 minutes of the first half though as Myler added another three points and veteran centre May crossed the whitewash twice in the space of two minutes.
The 34-year-old showed good pace to first exploit an overlap and touch down and then find a gap in the opposition defensive line an beat several players on his way to the try-line.
Burrell added a third try for the Saints early in the second half as well as Welsh failed to stop him after a typically bulldozing run from Brian Mujati had laid the platform.
Northampton's quest for a bonus point was momentarily derailed as Alex Waller was sent to the sin bin and the home side's pack took advantage of the extra man to give number eight Daniel Browne the chance to score.
Scottish fly-half Gordon Ross added the conversion but he was given a yellow card himself moments later and that gave the visitors just the boost they needed.
The Saints had to wait until just a few minutes from time but the powerful Burrell stepped up again when it mattered to seal what could be a crucial bonus point in the club's attempt to make the playoffs.
For London Welsh:
Tries: Scott, Browne
Cons: Davies, Ross
Yellow Card: Ross
Tries: May 2, Burrell 2
Pens: Myler 3
Yellow Card: Waller
London Welsh: 15 Tom Arscott, 14 Seb Stegmann, 13 Sonny Parker, 12 Seb Jewell, 11 Nick Scott, 10 Gordon Ross, 9 Alex Davies, 8 Daniel Browne, 7 Michael Hills, 6 Julio Cabello Farias, 5 Kirill Kulemin, 4 Matt Corker, 3 Paulica Ion, 2 Greg Bateman, 1 Tom Bristow.
Replacements: 16 Dan George, 17 Arthur Joly, 18 James Tideswell, 19 Adam Brown, 20 Adam Balding, 21 Rob Lewis, 22 James Lewis, 23 Tom Voyce.
Northampton Saints: 15 Ben Foden, 14 Ken Pisi, 13 George Pisi, 12 Luther Burrell, 11 Jamie Elliott, 10 Stephen Myler, 9 Lee Dickson, 8 Samu Manoa, 7 Phil Dowson, 6 Tom Wood, 5 Christian Day, 4 Courtney Lawes, 3 Brian Mujati, 2 Dylan Hartley (c), 1 Soane Tonga'uiha.
Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Paul Doran Jones, 19 GJ van Velze, 20 Calum Clark, 21 Martin Roberts, 22 Ryan Lamb, 23 Tom May.
Referee: Greg Garner