The bombshell news dropped on Monday with Red Bull confirming they had launched an investigation against the man who has orchestrated the Austrian drinks company's phenomenal success on the track.
Since Horner turned his back on trying to make it as a racing driver he found his true vocation in management.
Appointed as the youngest team principal in the Formula 1 paddock aged 31 in 2005 he has overseen six constructors championships and seven drivers titles - a remarkable achievement by any yardstick.
But with the launch of the car he hopes will deliver Max Verstappen a fourth straight title scheduled for next Thursday, he has now had attention diverted to preparing his defence for Friday's hearing led by an independent barrister.
The charge is serious.
On Monday Red Bull said: "After being made aware of certain recent allegations, the company launched an independent investigation.
"This process, which is already underway, is being carried out by an external specialist barrister.
"The company takes these matters extremely seriously and the investigation will be completed as soon as practically possible. It would not be appropriate to comment further at this time."
Horner issued a strong rebuttal, telling Dutch paper De Telegraaf: "I completely deny these claims."
The inquiry could not come at a worse time for the man who has become an ever-present fixture in F1's landscape over nearly two decades.
Power and sway
Eloquent, bright, combative, his spats with his opposite number at Mercedes Toto Wolff have made for compelling viewing, not least when Verstappen controversially deprived Lewis Hamilton of an unprecedented eighth world crown on the last lap of the last race of the 2021 season in Abu Dhabi.
As team principal and chief executive of Red Bull Racing, Horner has an enormous amount of power and sway over a vast empire based at the team's Milton Keynes headquarters in England.
During his time at the tiller, the company's workforce has ballooned from 450 to 1,500, with one of that number's allegations shocking the F1 community.
Williams chief James Vowles has been one of Horner's few peers to speak publicly about the case this week.
Without going into the specifics of the allegations, Vowles told Bloomberg on Monday: "All I can say is that should this ever happen in our regard, we'll be entirely supportive in terms of fixing it and making sure we have a culture that is accepting of everyone.
"I think it means we all have to look each other in the mirror and make sure that we are posing the right questions internally and acting in a way that we can only be proud of, not today but in the next 10 years."
Horner's ties with the ambitious Austrian company Red Bull date back to his time in F3000 and to Dietrich Mateschitz.
Mateschitz, the father of Red Bull who died in 2022, had purchased the Jaguar F1 outfit in 2004 - and saw enough in the young Horner to appoint him as team boss for 2005.
Among the many inspired moves Horner made was to bring on board Adrian Newey, ranked as one of the most talented engineers and designers of his or any other generation.
Newey, who climbed aboard the Red Bull wagon in 2006, produced the cars that won the drivers' and constructors titles every year from 2010 to 2013, the drivers' championship in 2021, and both championships in 2022 and 2023.
Newey's RB19, which ran riot on the track last year, is statistically the most successful car in F1 history, winning 21 out of 22 races.
And Red Bull will be mindful that Newey's contract is linked to Horner's which means if one leaves then the other is free to follow.
Despite all the attendant stress of being at the helm of steering such a colossal enterprise through the choppy waters of a multi-billion pound industry, Horner retains an air of boyish enthusiasm.
He married Geri Halliwell, Ginger Spice of the former Spice Girls pop band in 2015, and the couple have one son, Montague. Horner also has a daughter from a previous relationship.
A supporter of Coventry City football club, Horner is the longest-serving team principal in the paddock.
During that tenure he has inevitably had to deal with crisis after crisis - but none more serious than the one he is facing on Friday.
