He was in a same-sex relationship with James VI and I, and once intrigued with Cardinal Richelieu to use English ships against French Protestants.
But the painting of this colourful 17th century high society character - “found” in the south side’s Pollok House - has never attracted much attention in modern times ... until now.
The moustachiod dandy in question is George Villiers, 1st Duke of Buckingham, who is widely described as “James VI’s last lover”, and the portrait was considered to be a copy of a work by Flemish Baroque painter Sir Paul Rubens.
Now, in one of the rare events which truly astonish the art world, it has been proved to be genuine ... and is worth “tens of millions of pounds”.
Once again the experts have been proved to be disastrously wrong, but the painting’s true provenance might have remained a secret forever had it not been studied in detail by “art detective” Dr Bendor Grosvenor.
The whole fascinating story will be revealed in Thursday’s instalment of the BBC series Britain’s Lost Masterpieces, in which it’s explained how many of the trademark techniques associated with Rubens were hidden under layers of grime and “overpaint”.
In an arty parallel of advanced crime forensics it will also detail how the painting was finally “nailed” as a genuine Rubens, dating to the 1620’s.
The good news for Glasgow is that in one remarkable surprise windfall it has added a world class masterpiece to a collection which already includes pricless works by Rembrandt, Vermeer - and Dali.
Pollok House is run by the National Trust for Scotland, but the art collection is the responsibility of Glasgow Museums.
It appears the Duke of Buckingham painting was purchased in the late 19th century by Sir William Stirling Maxwell, who owned Pollok House - so all those years later it is still in the “right” location.
However unsurprisingly Glasgow Museums want to use it as a crowd-puller at the flagship Kelvingrove gallery in the city’s West End, and it will be available to view (or queue for) on Thursday, the day following the BBC Four programme.
Reports imply Kelvingrove may be its permanent home, but some south siders may argue it should stay in Pollok House, where it hung in a dining room for many years.
Instead it seems likely to be the centre of attention in Kelvingrove - which has just loaned out its current most famous painting, Dali’s Christ of Saint John of the Cross.
Karen Cornfield, property manager of Pollok House said: “This is such exciting news for Glasgow and shows what a great eye Sir William Stirling Maxwell had. Pollok House is packed with beautiful paintings that he collected over the years.
“We hope the BBC programme will encourage visitors to come to Pollok, explore the collection and to learn more about the family who brought so many great artworks to Glasgow.”
David McDonald, chairman of museums service operators Glasgow Life, said: “Unsurprisingly, we are beyond delighted to discover the painting is by Rubens, an artist renowned globally as one of the most important painters in history.
“Glasgow is proud of its extensive art collection, considered by many to be amongst the finest in Europe. Glasgow Museums strive to learn more about the artwork in our care and it has been a joy to work with Britain’s Lost Masterpieces to understand much more about this remarkable painting.
“We are excited to give as many people as possible the opportunity to see Rubens’ masterpiece in person. George Villiers, First Duke of Buckingham is sure to become one of the undoubted highlights of any visit to Kelvingrove.”
There is no precise estimate available (Antiques Roadshow style) for what the painting might fetch at auction, because it is officially “priceless” and, say Glasgow Museums, will never be sold.
The Duke of Buckingam was a flamboyant character whose meteoric success under James VI and I’s patronage fizzled under Charles I.
He led a series of disastrous military expeditions, became hugely unpopular, and on August 23, 1628 (aged just 35) was stabbed to death in The Greyhound pub in Portsmouth - where he had been trying to organise another campaign.
His assassin, who was hanged, was hailed as the equivalent of a martyr by the public - who had come to hate Buckingham.
However as a fictionalised romantic character he appears in Alexandre Dumas’ classic The Three Musketeers (1844), in which he is described as “the handsomest gentleman and the most elegant cavalier of France or England”.