Chester-born White impressed in a trial spell at The Stoop and the 19-year-old has joined the successful Quins academy.
The fly-half started out at Whitchurch RUFC before attracting the attention of Worcester Warriors at the age of 14 and, after completing his GCSEs, landed a scholarship at the renowned Bromsgrove School.
Worcester decided against offering White a contract in January but he captained his school to victory in the NatWest Schools Cup, leading to trials at Saracens and Harlequins.
He helped Quins reach the finals of the Singha Premiership Rugby 7s Series and was subsequently offered an academy contract as well as being dual-registered with Dorking RUFC.
White said: "I’m delighted to have signed for Harlequins; a club with such high calibre players, coaching staff and with such a great team culture.
"Playing alongside seasoned internationals and fly-halves with lots of experience like Nick Evans can only help in my development and motivate and drive me to reach the highest of standards."
Harlequins academy manager Tony Diprose added: "We are really pleased to welcome Luke into the Harlequins Academy set up this season.
"Luke has impressed us both with his attitude and his skill set during his trial period in pre-season. Both of these attributes were also on show in his performances in the Singha 7s series.
"He is a very ambitious and motivated young player and we are looking forward to overseeing his development at Harlequins and Dorking via his dual registration."
Helsby well beaten as Ellesmere Port reach Cheshire Plate final
Helsby fell to a 34-0 defeat at home to Ellesmere Port in the semi-finals of the Cheshire Plate. Port applied lots of pressure in the first-half and proved too strong for the hosts, although Helsby fared much better in the second period and tested the visitors line but finished scoreless.
Following a week’s break from league action, Chester RUFC return to business with a game against fifth-placed Leicester Lions at Hare Lane in National League Two North. Christleton, third in Division Three South, host leaders Bowdon and Helsby entertain Winnington Park 4ths in Division Five South (all 2.15pm).