Thomas Keneally's Commonwealth of Thieves is a popular history of the first years of the British colony in Australia, published in 2006.

Keneally (an Australian who is, of course, best known as a novelist, and as the author of Schindler's List) uses the term "Tawny Prince" – always with capitals – five times in the course of his book. The most significant mentions are:

"... In Spitalfields to the east, in squalor unimaginable, lived all classes of criminals, speaking a special criminal argot and bonded together by devotion and oath to the criminal deity, the Tawny Prince. The Tawny Prince was honoured by theft, chicanery and a brave death on the gallows..." [p.20]

[Of convicts on their arrival in Australia:] "Not that they were reborn entirely, since they brought their habits of mind and the Tawny Prince, the deity of the London canting crews, with them" [p.81]

[Of a wild celebration in the rain:] "The great Sydney bacchanalia went on despite the thunderstorm. Fists were raised to God's lightning; in the name of the Tawny Prince and in defiance of British justice, the downpour was cursed and challenged..." [p.89]

All this is referenced, so Keneally did not invent the Tawny Prince, but a little further research does suggest he took a fairly basic reference, elaborated it, embroidered it, and used it to produce a much more solid and distinct figure than the evidence actually warrants. All in the name of good colour, I am sure.

Let's start with Keneally's own notes. He cites as his references for a collection of material about the "Tawny Prince" and cant (thieves' slang) as Watkin Tench's Sydney's First Four Years and Captain Grose's Dictionary of the Vulgar Tongue of 1811.

Tench was an officer in the marines who was part of the First Fleet. The book Keneally cites was a 1961 reprint of one originally titled A Narrative of the Expedition to Botany Bay, first published by Debrett in London in 1789. This contains no reference to the Tawny Prince, so in fact Keneally's only source is Grose (1731-91), an antiquary, whose work (correctly titled Classical Dictionary of the Vulgar Tongue) was first published in 1785.

This work does contain a passing reference to the Tawny Prince, not in the form of a separate entry, but rather inserted as a phrase that forms part of a much longer oath supposedly taken by "Gypsies" (a term which Grose uses not to mean "Romani," but as a synonym for vagrants of all sorts) when "a fresh recruit is admitted into the fraternity." The relevant extract is the first of several clauses, and is:

"I, Crank Cuffin, do swear to be a true brother, and that I will in all things obey the commands of the great tawney prince, and keep his counsel and not divulge the secrets of my brethren."

Now, The Routledge Dictionary of Historical Slang confirms that a "crank-cuffin" is an 18th century term for a vagrant feigning sickness, which at least implies that the claimed oath is in the language of the period, but Grose incorporates no commentary at all, so we are left to our own devices in attempting to make sense of the terms and of the passage as a whole.

We can start with the bio of Grose that appears in the Dictionary of National Bibliography, which notes:

From 1783 he published in a torrent to make a living. The Supplement to the Antiquities was resumed, with a greater proportion of views from other artists, particularly S. H. Grimm, and was completed with 309 plates in 1787. This and the main series were reissued in a cheaper edition in 1783–7. A Classical Dictionary of the Vulgar Tongue (1785) and A Provincial Glossary, with a Collection of Local Proverbs, and Popular Superstitions (1787) were at the time the largest assemblage of ‘non-standard’ words or meanings, about 9000, omitted from Samuel Johnson's Dictionary; they drew on his fieldwork as far back as the 1750s. The first parts of two other pioneering works appeared in 1786: Military Antiquities and A Treatise on Ancient Armour. Both relied mainly on his specialist library and the armouries at the Tower of London, but also included observations on military music from the 1740s. Of more popular appeal was Rules for Drawing Caricaturas: with an Essay on Comic Painting (1788).

How much further does this get us? The reference to "fieldwork" is intriguing, but it's balanced by the discussion of a "torrent" of works churned out to make a living, and in fact a careful search shows that Grose's source was not some vagrant informer, but rather the grammarian James Buchanan's New Universal Dictionary of 1776, which has an entry for "Gypsies" that contains a fuller version of the same oath that Grose gives, referenced more precisely to what appears to be a description of the Romani people, in which is embedded in a significantly more detailed account of gypsy oath-making. Buchanan, sadly, gives no source for his information or the reference to the "tawney prince", but his own gloss on the oath as a whole is as follows:

"The Canters have, it seems a Tradition, that from the three first Articles of this Oath, the first Founders of a certain boastful, worshipful Fraternity, who pretend to derive their Origin from the earliest Times, borrowed of them, both the Hint and Form of their Establishment. And that their pretended first derivation from Adam, is a forgery..."

That is as far back as I have been able to trace the term, but I'm afraid that a more sober consideration of Grose and especially of Buchanan and his gloss indicates that the "great tawny prince" was not some sort of special deity of thieves, in the way that Keneally uses the term, but simply a synonym for the prince of darkness – that is, the devil. (Note, in support of this argument, the lack of capitals in the term "tawney prince" as given by both Grose and Buchanan, and in contradistinction to Keneally's usage.)

As such, it seems likely that the oath taken is presented not as one sworn to a real "god" of any sort, but rather an inversion of the sort of decent oath an honest Christian might swear by his or her God, in which insertion of mention of the devil actually serves to underline the dastardly and perverted nature of the oath for the dictionary's intended audience – not thieves, but gentlefolk who, it is intended, will be horrified by it. The idea that thieves and criminals of every stripe were organised into an ordered fraternity that placed itself in distinct opposition to decent society was not only an outrage in itself, but also helped to justify their persecution – which, in this period, before the repeal of the 'Bloody Code', was notoriously severe.

tldr: Keneally has misunderstood the reference, and gone on to build a definite, distinct and special "deity of the London canting crews" – one honoured by "theft, chicanery and a brave death on the gallows" – on that misunderstanding.

Providing the title and author of the book where you encountered this claim might help you get an answer to this.