The England centre touched down for the opening try and he played a key role in scores by David Paice and Topsy Ojo as London Irish turned the Premiership table on its head.
Jamie Gibson wrapped up the Exiles' first bonus point of the season with their fourth try to cap another outstanding individual display to leave Northampton's unbeaten record in tatters.
By the time Ken Pisi and then Phil Dowson scored late consolation tries for Northampton, the Saints already trailed 33-3 and their goose had been well and truly cooked.
There was nothing to indicate a London Irish victory, let alone a performance like this, was on the cards from a side languishing 11th in the Premiership.
The Exiles defence had conceded an average of 36 points a game up until today, with only Sale suffering a worse start to the campaign.
In complete contrast, Northampton arrived in Reading as guardians of the only unbeaten record left in the competition but they were without injured captain Dylan Hartley.
Tom Wood is not back to his best and Courtney Lawes, starting at blindside flanker, was overshadowed by a dominant effort from the London Irish pack.
With that platform, the Exiles back division were able to shine and no-one shone brighter than Joseph, who made his England Test debut on the summer tour of South Africa.
The England coaching staff may be back in Johannesburg this weekend to watch the Springboks play New Zealand but it will not be long until reports of Joseph's performance reach them.
Both Stephen Myler and Steven Shingler had missed penalty shots at goal in a point-less first 24 minutes when Joseph created and then scored the first try.
London Irish snaffled an overthrown Northampton lineout and moved the ball wide for Joseph to stab a grubber kick through for Marlande Yarde to kick on.
Vasily Artemyev, Northampton's stand-in full-back, failed to cover the bouncing ball and Joseph followed up to score his second try of the season.
London Irish were unrelenting for the rest of the half. Prop Halani Aulika was stopped just before the line and number eight held up over it as Ofisa Treviranus hammered away at the line.
Shingler extended the Exiles' lead with a penalty before Joseph hit the line at blistering pace inside the Northampton 22 and, two phases later, Paice twisted over the line to score.
Ian Humphreys' 40-metre drop-goal handed London Irish an 18-0 lead at half-time. Northampton had to respond and they showed early intent after the restart with a penalty from Myler.
A brilliant run from Pisi had London Irish on the back foot and Myler almost worked an opening for Noah Cato but they could not find a way through.
And London Irish then hit Northampton with a brilliant third try, finished by Ojo but created by Joseph as the Exiles attacked from clean lineout ball.
Joseph beat Myler for pace and then drew two defenders before slipping a delightful pass to Ojo, who was tackled by Pisi but had the momentum to roll over the line and score.
Shingler edged the Exiles further ahead with a penalty after a powerful driving maul before captain Bryn Evans came within inches of scoring the fourth try after collecting a one-handed pass from Joseph.
But the Exiles did not have to wait long. From a quick lineout, Shingler swung a long pass to Treviranus, who held off two defenders and sent Gibson racing away for the try.
Northampton, to their credit, kept plugging away with scrum-half Lee Dickson particularly lively, and they worked a try for Pisi in the right corner.
Dowson burrowed over a second Northampton try but Shingler added two more late penalties as London Irish rounded off their best performance of the season with a memorable victory.
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