Quins cut loose against the Reds at Twickenham Stoop scoring eight tries in a crushing 40-18 win.
The Londoners had the game wrapped up by half-time after Dorn’s first two tries and further scores from Rob Purdham and John Wells opened up a 24-0 interval lead.
Quins boss Brian McDermott said that he was happy that the Londoners’ five departing players were given a good send off.
"It was important for me that we gave the players leaving us the send-off they deserved” said McDermott.
"I was delighted with their attitude and the way the game went for us – we've comfortably beaten a good side.
"All the players leaving will be replaced and I will start to build for next season when I get into the office on Monday.
"I'm pleased that we won and ended the season on a high because it is long winter if you end with a defeat."
Reds coach Karl Harrison admitted that his side had not performed to expectations, and that their position in the play-offs was now out of their hands – dependent on Warrington’s result against St Helens on Sunday.
"Harlequins wanted to win more than us and we were very poor” Harrison commented.
"We had too many players who were off their game and that's very disappointing – we can't afford to play like that in the play-offs.
"This loss now means that it is out of our hands where we finish and who we play – I didn't want that."