Johnston replaced long-time chief Peter Doust in August, and he has been forced to guide the club through a tricky period in his 15 months in charge.

From a disastrous on-field performance this season to the Jack de Belin sexual assault allegations off-the-field, it has not been a positive year for the Dragons.

Johnston will remain in charge until the de Belin court proceedings conclude, with the NSW forward set to face trial in early February.

One of Johnston's last acts as CEO has been the implementation of an end-of-season review by Phil Gould, which led to eight staff changes in the team's football department.

In announcing his departure, Johnston acknowledged the past year has been difficult and said he's looking forward to finishing his tenure on a positive note.

"The past 12 months as CEO of St George Illawarra has been a challenging yet enjoyable experience," Johnston said.

"I have made this decision to focus on my family and the farm back home in Dunedoo.

"I look forward to the next four months overseeing an exciting pre-season following the completion of the club's extremely important football department review."

Johnston shifted from the role of chairman to CEO after WIN bought into the joint venture. That saw Andrew Gordon become chairman.

Read more: Berry trainer prepared as NSW braces for 'catastrophic' fire conditions

Gordon praised Johnston for his management of the club during what has been an eventful period.

"Brian has overseen and played a vital role in what has been a vast period of change for the St George Illawarra club," Gordon said.

"Brian has faced significant challenges both on and off the field in his time back in charge of the Dragons, and all the while navigated a new ownership model and a long-term predecessor in a competent manner.

"While his second tenure as chief executive officer was brief, Brian's successor will be better placed because of his contribution to the club; he will always be a part of the Dragons family."

While you're with us the Illawarra Mercury is offering sport readers 20% off an annual digital subscription. Sign up to stay up to date with all the local sports you love for only $3.00 a week. Terms and conditions apply.