London Irish left Twickenham on Saturday with a loosing bonus point as Harlequins secured the victory with tries from Ugo Monye and Ollie Lindsay-Hague.
Irish were slow out of the blocks in the first half, struggling to retain possession, and though preventing the visitors from scoring a single point after the break, were unable to reclaim the deficit
The penalty count gave Harlequins a heavy advantage in the first quarter and it showed come half time with the scores at 6-20 to the visitors.
Shane Geraghty kicked all of the Exiles 15 points, and made an impressive effort from within his own half that recoiled back off the crossbar.
Early worries for the home side as Tom Homer had to be replaced by Tom Fowlie, pushing Topsy Ojo out to full-back for the remainder of the game.
Quins were first to register points when Monye opened the scoring out wide after 11 minutes despite the efforts of Fowlie to get across.
Good defence by the home team got them out of a few sticky situations but the referee became frustrated with ill-discipline and sent Eammon Sheridan to the bin for holding on.
Replacement winger Lindsay-Hague took advantage of the extra man running in the score, and it was to be the visitors last score of the game.
Irish came out with a vengeance in the second half and the decisions started going in their favour.
Geraghty made it three kicks from four, and then closed the gap again to just eight points moments later.
Though possession still favoured Harlequins, they were unable to break through and with just 15 minutes remaining; the Twickenham crown had a match on their hands as Geraghty made it a five point game.
A fully replaced front row showed the instrumental value of Exile props Tom Court and Halani Aulika, who dominated a strong Irish set piece for most of the game before being replaced by Matt Parr and Leo Halavatau.
A Nick Evans Penalty at scrum time inside the Irish 22 sailed wide, like a number of his place kicks throughout the match.
The clock went red, and the home side had the full pitch to cover before they could think about scoring a winner. A probing kick behind the defensive line nearly put Fergus Mulchrone through but Mike Brown scrambled it into touch to bring the match to a close.
A tightly fought contest that displayed great talent and a lot of promise for London Irish. Not namely the defense that stayed solid through the entire second period.
The Exiles will now prepare for Newcastle Falcons at Kingston Park next week.
Score sequence (London Irish first): 0-3, 0-10, 3-10, 3-13, 6-13, 6-20, 9-20, 12-20, 15 - 20
Scorers: Harlequins: Penalties: Evans (8, 28); Tries: Monye (11), Lindsay-Hague (35); Conversions: Evans (2); London Irish: Penalties: Geraghty (26, 35, 43, 55, 67)
15. Tom Homer (rep: Tom Fowlie, 7, Rep: Gerard Ellis, 78); 14. Topsy Ojo; 13. Fergus Mulchrone; 12. Eamonn Sheridan (rep: Chris Noakes, 67); 11. James Short; 10. Shane Geraghty; 9. Scott Steele (rep: Tomås O'Leary, 50); 1. Tom Court (rep: Matt Parr 57); 2. David Paice; 3. Halani Aulika (rep: Leo Halavatau, 54-64, 75); 4. George Skivington (Captain); 5. Dan Leo (Rep: Conor Gilsenan, 79); 6. Blair Cowan; 7. Ofisa Treviranus; 8. Tom Guest (rep: Kieran Low, 60)
Replacements: 16. Gerard Ellis; 17. Matt Parr 18. Leo Halavatau; 19. Kieran Low; 20. Conor Gilsenan; 21. Tomås O'Leary; 22. Chris Noakes; 23. Tom Fowlie
15. Mike Brown; 14. Marland Yarde; 13. Matt Hopper; 12. Jordan Turner-Hall; 11. Ugo Monye (rep: Ollie Lindsay-Hague, 28); 10. Nick Evans; 9. Danny Care; 1. Joe Marler (Captain); 2. Joe Gray (rep: Rob Buchanan, 44); 3. Kyle Sinckler; 4. Charlie Matthews (rep: George Merrick, 60); 5. George Robson; 6. Luke Wallace (rep: Jack Clifford, 65) (rep: Mark Lambert, 48-54); 7. Chris Robshaw; 8. Nick Easter
Replacements: 16. Rob Buchanan; 17. Mark Lambert; 18. Will Collier; 19. George Merrick; 20. Jack Clifford; 21. Karl Dickson; 22. Ben Botica; 23. Ollie Lindsay-Hague
Referee: Greg Garner