A long time ago

This may be simply a US expression, but it describes anyone who is just a mean son-of-a-gun. (being PC here) The comparison with the junkyard dog refers to the guard dogs used to patrol wrecked auto parts lots where the losses to midnight thievery would be great without the dogs presence. They are usually Dobermans or German Shepards, and are bred and trained to be rather intimidating and downright dangerous.
Is this used elsewhere in the English speaking world?

My old friend wrote this song years ago and mentioned your 'junkyard dog.'

Leroy Brown by Jim Croce

Well the South side of Chicago
Is the baddest part of town
And if you go down there
You better just beware
Of a man named Leroy Brown

Now Leroy more than trouble
You see he stand 'bout six foot four
All the downtown ladies call him "Treetop Lover"
All the mens just call him "Sir"

And it's bad, bad Leroy Brown
The baddest man in the whole damn town
Badder than old King Kong
And meaner than a junkyard dog

Now Leroy he a gambler
And he like his fancy clothes
And he like to wave his diamond rings
In front of everybody's nose
He got a custom Continental
He got an Eldorado too
He got a 32 gun in his pocket for fun
He got a razor in his shoe

And it's bad, bad Leroy Brown
The baddest man in the whole damn town
Badder than old King Kong
And meaner than a junkyard dog

Well Friday bout a week ago
Leroy shootin' dice
And at the edge of the bar
Sat a girl named Doris
And ooh that girl looked nice
Well he cast his eyes upon her
And the trouble soon began
And Leroy Brown learned a lesson
'Bout messin' with the wife of a jealous man

And it's bad, bad Leroy Brown
The baddest man in the whole damned town
Badder than old King Kong
And meaner than a junkyard dog,

Well the two men took to fightin'
And when they pulled them from the floor
Leroy looked like a jigsaw puzzle
With a couple of pieces gone

And it's bad, bad Leroy Brown
The baddest man in the whole damn town
Badder than old King Kong
And meaner than a junkyard dog

I love that song. As soon as Ward mentioned this line, it was this Jim Croce song that sprang to mind.

I agree, it is known in the UK and Ireland, but solely because of that song. I doubt that anyone would actually use it in natural speech.

DFG

I was fortunate to see Jim Croce's only English concert and remember it to this day. A terrific writer and performer! What a great privilege to have known him, SR.
I don't think the phrase is common in England. We do have an interesting phrase about dogs, 'as fit as a butcher's dog'.

I'd forgotten that one Henry. My dad used to use that one too, obviously one of the many many legacies of our heritage (although only older generations - I doubt that my 11 year-old stepdaughter's ever heard it.) And I also agree with both Henry and David that we don't use the 'junkyard dog' saying here. If I say this to anyone they immediately think of that song.