London Irish failed to follow up last week's RDS victory with the Scarlets stunning their hosts 27-25 at the Madejski Stadium on Saturday.
Trailing 21-18 with 15 minutes to go, prolific wing Sailosi Tagicakibau ran in his seventh try of the season to give the Exiles sight of victory.
But with little over a minute remaining on the clock, substitute Peter Richards conceded a penalty in front of the posts and Jones landed the decisive kick.
Wales fly-half Jones was in commanding form, kicking magnificently throughout to finish with 17 points and showing composure to help Scarlets remain top of Pool Six.
Irish edged the try count three to two with Declan Danaher and Elvis Seveali'i also crossing, but victory could have gone either way in a compelling contest at the Madejski Stadium.
Both teams attacked at will with Daniel Evans' individual try the highlight for the Scarlets while winger Lee Williams also touched down in a see-saw encounter.
It looked ominous for Scarlets when Irish scored with their first attack of the match, a try instigated by Seilala Mapusua's bulldozing run through midfield.
Mapusua broke two tackles and the ball was recycled to number eight Chris Hala'ufia, who supplied fellow back row Danaher the scoring pass with just six minutes on the clock.
When Lamb landed a drop goal shortly after, Irish looked comfortable leaders but Scarlets hit back with a moment of individual brilliance from Evans.
The full-back chipped over the Exile's advancing line, gathered and raced around Hewat before winning the 25-metre race to the line.
Jones' conversion and a penalty sandwiched three points from Lamb, but once again Irish pulled clear with an all-Samoan try though Seveali'i.
Once again Mapusua was the creator, only this time using his boot to chip through to Seveali'i who pounced on the kind bounce and touched down.
The ebb and flow continued as Scarlets replied two minutes later when Williams escaped a congested left wing and exposed Hewat's lack of pace with a crafty chip that he got fingertips to in the left corner.
Homer was shown a yellow card for taking Williams out in the air and when Jones slotted a penalty, the sides entered half-time all-square at 18-18.
Scarlets should have taken the lead shortly into the second half, pulverising their opponents at a scrum and winning two more before centre Jonathan Davies almost danced over.
Somehow Irish, after coming under immense pressure, wrestled the ball free from Davies and escaped.
But the momentum had clearly shifted and when Jones added his third penalty, Scarlets were in the driving seat.
Slowly, however, Irish clawed their way back into contention and were inches short of crossing through Tagicakibau following a brilliant pass by Lamb.
Jones struck the uprights with a penalty and in the 66th minute the Exiles landed what looked a telling blow.
Seeking to exploit space on the right, Evans' pass intended for Mark Jones was intercepted by Tagicakibau who had stationed himself in the Llanelli line.
The Samoan still faced a 75-metre run to the line but once in the clear no player had the gas to stop him as he crossed beneath the posts with Lamb converting.
Tension continued to grip the Madejski Stadium as Jones booted his fourth penalty before snatching victory with just over a minute to go.
For London Irish:
Tries: Danaher, Seveali'i, Tagicakibau
Con: Lamb 2
Tries: Evans, M Jones
Con: S Jones
Pen: S Jones 5
London Irish: 15 Peter Hewat, 14 Tom Homer, 13 Elvis Seveali'i, 12 Seilala Mapusua, 11 Sailosi Tagicakibau, 10 Ryan Lamb, 9 Paul Hodgson, 8 Chris Hala'Ufia, 7 George Stowers, 6 Declan Danaher, 5 Bob Casey, 4 Nick Kennedy, 3 Faan Rautenbach, 2 David Paice, 1 Dan Murphy.
Replacements: 16 Danie Coetzee, 17 Clarke Dermody, 18 Paulica Ion, 19 Andy Perry, 20 Richard Thorpe, 21 Peter Richards, 22 Chris Malone, 23 Alfredo Lalanne.
Scarlets: 15 Daniel Evans, 14 Lee Williams, 13 Sean Lamont, 12 Jon Davies, 11 Mark Jones (capt), 10 Stephen Jones, 9 Martin Roberts, 8 David Lyons, 7 Dafydd Jones, 6 Simon Easterby, 5 Dominic Day, 4 Lou Reed, 3 Deacon Manu, 2 Matthew Rees, 1 Iestyn Thomas.
Replacements: 16 Ken Owens, 17 Phil John, 18 Jamie Corsi, 19 Vernon Cooper, 20 Richard Pugh, 21 Tavis Knoyle, 22 Rhys Priestland, 23 Rob Higgitt.
Referee: Peter Fitzgibbon (Ireland)
Assistant referees: Paul Haycock (Ireland), Dermot Moloney (Ireland)
Television match official: Laurie Bryant (England)
Assessor: Patrick Thomas (France)