Recent signings like winger Brian Tuohy, lock Bernie Upton and number 8 Mike McComish lined out for the westerners for the first time, with young developments players, Tiarnan O'Halloran and Eoin Griffin, looking right at home when introduced as second half substitutes.
Another debutant, out-half Miah Nikora, kicked the visitors into an early lead and the former New Zealand Under-21 international landed three penalties in all in what was a well-contested but scrappy encounter.
All three kicks were in the first half as Harlequins turned around with a 10-9 lead, thanks to a George Lowe try and a penalty and conversion from Rory Clegg.
Clegg touched down for 'Quins, shortly after the restart, and while a try for newcomer Tuohy, off a well-executed back-line move, set up an exciting final quarter, the home side managed to hold on for the win.
CONNACHT: Troy Nathan; Brian Tuohy, Niva Ta'auso, Aidan Wynne, Fionn Carr; Miah Nikora, Conor O'Loughlin; Ronan Loughney, Adrian Flavin (capt), Robbie Morris, Michael Swift, Bernie Upton, Mike McCarthy, Johnny O'Connor, Mike McComish.
Replacements: Andrew Browne, Kieran Campbell, Eoin Griffin, Jamie Hagan, Conor Higgins, Dermot Murphy, David Nolan, Ray Ofisa, Tiarnan O'Halloran, Brett Wilkinson.