In an on-air announcement on Thursday, Hadley told 2GB listeners he was ending his 43-year career behind the mic as a race caller, rugby league commentator and morning-show host.
The 70-year-old said he had been in talks with management at station owner Nine about his future for a while before notifying of his decision to quit in the previous week to spend more time with his seven grandchildren.
"I started to think (about) how long I've got left on this earth and do I want to keep getting up at half past three in the morning, meaning I go to bed early at social events - I'm always first out the door," he said.
"I don't want to be the first out the door anymore."
Hadley joined rival station 2UE in 1986 as a rugby league commentator and later worked as a fill-in talkback presenter for the dominant John Laws before moving to 2GB in 2001.
He took over the morning program there a year later and went on to dominate Sydney radio ratings for the slot with his frequently outspoken views.
His morning program is syndicated to stations across much of regional NSW and is also broadcast on Brisbane's 4BC and various regional Queensland networks.
"It's been a hell of a ride from a young bloke wanting to call the races to being the old bloke sitting in this particular chair in the study ... but the time has come for someone else to do the job," Hadley said.
The conservative broadcaster's influence is coveted across the political spectrum, while his program features regular interviews with Opposition Leader Peter Dutton.
He also heads 2GB's Continuous Call team, which features rugby league discussion and commentary and is syndicated nationally.
He was inducted into the Australian Commercial Radio and Audio Awards hall of fame in 2017, while he was awarded a Medal of the Order of Australia in 2002 for services to rugby league as a broadcaster and for his charity fundraising.
But Hadley has also been the subject of controversy during his long career, including in 2013 when he was briefly suspended over allegations he verbally abused an employee.
Similar bullying allegations were aired six years later, prompting an on-air apology from the talkback host, who admitted to "shortcomings" earlier in his career.
Hadley is due to host his last morning show on December 13.
But the 70-year-old said he was not retiring from work completely, saying he planned to "bob up somewhere" in 2025 - although it would not be a full-time post and "certainly not on radio".
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