Following a glittering playing career that included three Grand Final wins, two Challenge Cup Final victories and international honours, McDermott began coaching with Huddersfield Giants before joining Leeds Rhinos as Tony Smiths assistant.
McDermott was appointed head coach of Harlequins Rugby League in July 2006 and in his first two months, successfully steered the club away from the relegation zone.
Always a strong believer in home-grown talent, McDermott oversaw a development programme which began regularly producing London Super League stars.
In 2008, Londoners Tony Clubb and Louis McCarthy Scarsbrook both made their England debuts and have since made further international appearances.
Recent seasons have seen the emergence of further regular Super League starters produced from within including Jamie O'Callaghan, Ben Bolger, Olsi Krasniqi, Lamont Bryan, Dave Williams to name but a few.
Earlier this season, McDermott fielded squads featuring only one overseas player over three successive rounds including a victory against Bradford and picked an all-British starting 13 against Huddersfield in July.
McDermott said today:
"I have really enjoyed my four years at Harlequins RL but now feel the time is right time hand over the reins. I want to spend more time with my family who are based in Yorkshire and this opportunity with The Rhinos is very exciting.
"I am honoured and proud to have been a part of this great clubs history and leave in the knowledge that Harlequins RL will soon reap the rewards of the hard work and quality coaching taking place throughout London and the South East.
"I have been fortunate to work with some great people during my time here, no one more so than our Chairman and owner, David Hughes. His outstanding commitment and financial backing of this club is only been equalled by his recent show of integrity and decorum in very testing and difficult circumstances.
"When the season is over I will say some difficult farewells but until that time comes, I will give everything I have and hope to leave the club, and London rugby league, in the best possible state."
Harlequins Rugby League chairman, David Hughes, thanked McDermott for his contribution to the club:
"The role of head coach at Harlequins RL is hardest job in Super League. It is a unique combination of coaching an elite team in a fiercely competitive league and being at the forefront of promoting and developing our sport in the capital.
"The Club has made significant progress under Brian's stewardship and he has created a blueprint for how Development Clubs invest in, and nurture local talent
"With his commitment to, and his passion for London rugby league, Brian has done a fantastic job at Harlequins RL and leaves with our full understanding and best wishes. We look forward to our first game against him next season.
"We will now begin the process of appointing a successor who will continue the clubs commitment to fostering and developing local talent to ensure the healthy future of Super League in London."