Very little is known of Sydney Carton's past other than the fact that he has spent it in dissipation.  In Chapter 4 of Book the Second of A Tale of Two Cities, after Carton saves Darnay from the gallows because of the their resemblance having been the key to Darnay's acquittal, the two men encounter each other by chance on the street and go into a tavern.  Then, after Darnay leaves, Carton regards himself in a mirror on the wall, "Do you particularly like the man?" he mutters to his image, telling it that Darnay reminds him of what he has "fallen away from" and what he might have been. 

Sydney Carton "would never be a lion, [but] he was an amazingly good jackal."  Carton is the brains behind hislegal partnership with Styrver.  In Chapter 11 of Book the Second, Stryver keeps Carton up half the night working on a legal case. In an example of dramatic irony, Stryver tells Carton that he has been ashamed of Sydney.  Sydney replies,

"It should be very beneficial to a man in your practice at the bar, to be ashamed of anything...you ought to be much obliged to me. 

When Stryver urges Carton to better himself, Sydney says that he is incorrigible and he has "no business to be, at all, that I know of."  Clearly, Sydney Carton has a very low opinion of himself for his drunkenness and his having allowedhis great mind to be preyed upon by Stryver when in his youth he should have excelled far beyond Stryver, who has merely "shouldered his way through life."

It is interesting to note that while Sydney Carton is around forty years old when he dies, he seems younger because of his youthful infatuation and idealization of Lucie Manette.  His love is a chaste love, pure and platonic.  His sacrifice for her is heroic, of course, but it is also a redemption for his life of dissipation.

In Charles Dickens' novel, A Tale of Two Cities, the reader is introduced to Sydney Carton.

Sydney Carton is a dissipated English lawyer who spends a great deal of his life drunk. Although he has a brilliant legal mind, his alcoholism keeps him from becoming a success.

Of himself Carton admits to Darnay:

I am a disappointed drudge, sir. I care for no man on earth, and no man on earth cares for me.

Carton and Stryver spend a great deal of time together, having known each other since their school days.

C. J. Stryver is the quasi-law partner of Sydney Carton. He makes his living by exploiting Carton's legal mind. Unlike Carton, Stryver is motivated and active, but he is also unprincipled...

In the discussion between the silent partners, Stryver brings up their past at school. We discover that during Carton's childhood he never had much ambition, and though he gives off an especially careless attitude about a great many things.

Stryver draws to Carton's attention his changeable moods in school, positive one day and dismal the next:

“The old Sydney Carton of old Shrewsbury School,” said Stryver, nodding his head over him as he reviewed him in the present and the past, “the old seesaw Sydney. Up one minute‚ and down the next; now in spirits‚ and now in despondency!”

It is apparent now that the law of motivation found in Carton has been his since his youth. Nothing has changed. He would be full of life one day, but depressed the next. He never dedicated himself to anything with fervor or passion.

Carton admits that while he did little of his own work (exercises), he rarely did his own. It is in this way that he still operates with Stryver. Stryver uses Carton's quick legal, intellectual acuity for his own cases, but Carton has never applied himself to his own career or success, and so assists quietly and drinks a great deal; and while he "would never be a lion, he was an amazingly good jackal." Stryver uses Carton in this way, and Carton is satisfied with his lot in life, or so he wants others to believe.

Stryver sums Carton up at the end of their conversation, and he seems to know Carton as well as the man himself:

“Carton,” said his friend, squaring himself at him with a bullying air...and the one delicate thing to be done for the old Sydney Carton of old Shrewsbury School was to shoulder him into it, “your way is, and always was, a lame way. You summon no energy and purpose.

Sydney Carton had responded earlier to another question that requires much the same answer:

'Ah!' returned the other, sighing: 'yes! The same Sydney, with the same luck. Even then I did exercises for other boys, and seldom did my own.'

And...

[Carton to Stryver:] You were always in the front rank, and I was always behind.

The conversation continues, and Carton, half-heartedly, blames Stryver for his own situation:

'Even when we were fellow-students in the Student-Quarter of Paris...you were always somewhere, and I was always—nowhere.'

'And whose fault was that?'

'Upon my soul, I am not sure that it was not yours. You were always driving and riving‚ and shouldering and pressing, to that restless degree that I had no chance for my life but in rust and repose.'

Carton's childhood was filled with a lack of direction, seemingly his own fault, though he humorously blames Stryver. It seems he knows he is the problem:

'God knows. It was my way, I suppose.'