After three successive league defeats it was a welcome four points for Northampton, who saw backs coach Paul Grayson leave in the week.
In atrocious conditions it was always going to boil down to a forward battle with Northampton's tries coming through a penalty try and Ken Pisi, though it was London Welsh's pack who made the first impression with a penalty try.
The visitors' maul trundled 25 metres from a lineout before it was hauled down under the posts. Referee David Rose awarded a penalty and sent Saints number GJ Van Velze to the sin-bin. Gordon Ross added the extras to make it 7-0 in as many minutes.
Ryan Lamb hit back with a calmly taken drop goal four minutes later after the home side nicked a Welsh lineout, but the visitors' maul was proving a handful even for Northampton's much-vaunted pack.
Another London Welsh lineout drive drew a penalty and from 45m Ross restored their seven point lead. Ross missed with another long range effort before Van Velze returned to the field.
At the second scrum of the game the Northampton pack had a huge drive that drew a penalty, with Phil Dowson - among the ensuing scuffle - making sure the London Welsh front row knew they were back in the game.
Suddenly the crowd and Saints were alive. From the lineout the home pack hit back with a retaliatory drive that almost galloped into the London Welsh 22 before Franck Montanella brought it down and was sin-binned.
With a forward down for a scrum on their five-metre line the visitors succumbed to the inevitable as they conceded a penalty try and Lamb drew Northampton level with the conversion after 27 minutes.
London Welsh were hanging on and a series of penalties saw them back defending another five-metre scrum. This time they had a full complement of forwards with Montanella back at loosehead and they drove a penalty out of Saints.
But the visitors had 15 players for barely a minute when they had a second player binned after Dan Caprice took out Jamie Elliott as the Northampton wing caught a high, hanging kick.
Saints took advantage with Lamb's vision giving them a second try. With advantage being played, Lamb's crossfield kick bounced perfectly into the arms of Pisi to score in the corner. Lamb converted to give Saints a 17-10 lead two minutes before half-time.
Lamb added three more points five minutes into what proved to be a dire second half.
Ross wiped that out on 55 minutes after Saints' lineout, which malfunctioned all afternoon, lost another throw on the visitors' 10m line and a sweeping move needed Elliott to sprint across to kick the ball out just a metre from his tryline.
After Welsh started the game with a huge maul, Saints ended it with one of their own, with the visitors lucky to concede just three points after a 20m lineout drive.
With seven minutes to go Ross landed a 35m penalty to give the visitors a deserved losing bonus point.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.