Michael Wilson, a 23-year-old Royal Irish Regiment soldier from Marlacoo Road in Tandragee and 24-year-old tree surgeon Gareth McKinney from Charles Park in Portadown were initially both charged with murdering Mr Smyth.
However, as their trial was about to enter its second day earlier this year, they were re-arraigned and each pleaded guilty to a charge of manslaughter.
McKinney was handed a six-year sentence, half of which will be spent in custody with the remaining three years on supervised licence upon his release.
Wilson - who committed four further violent offences following the fatal attack on Mr Smyth - was handed a seven-year sentence.
As he was deemed by probation staff to pose a significant risk of serious harm to others, Mr Justice Weir told him that after serving half of his sentence, his eligibility for release would be determined by the Parole Commission.
Wilson will also serve an extended three years on licence when he is released from custody.
Mr Smyth, who was 32, instigated a fight with Wilson in the early hours of June 2010 and after McKinney participated in the attack, Mr Smyth was found beside the Folley River in Armagh by a woman walking her dog.
Mr Smyth was rushed to hospital and was later transferred to a nursing home, where he spent two years in a vegetative state, before his family made the heart-breaking decision to withhold further treatment based on medical advice, on 12 June 2012.
A victim impact statement on behalf of Mr Smyth's mother revealed the toll her son's death has had on her, especially the decision to allow him to die and the physical effect caring for him had on her.
In the report, Mr Smyth's mother said: "Lee was not an angel and he had a lot of his own problems, but he was my only son and I miss him so much.
"He didn't deserve to be attacked and left in the way that he was."
A previous hearing at Armagh Crown Court, sitting in Belfast, heard that Mr Smyth had been involved in several incidents prior to the riverside confrontation with Wilson and McKinney.
Mr Smyth and his girlfriend attended a house party on 5 June, during which he assaulted a fellow reveller.
After the couple had gone home, a gang walked up to their house and an object was thrown at the property, causing Mr Smyth to get out of bed and give chase.
The court heard that Mr Smyth returned to the house and began searching for a weapon before leaving the house again.
Armed with a piece of wood, Mr Smyth then confronted a teenager in the Glenside area close to the Folley River.
Police intervened in this incident, which occurred at around 4.35am, with officers taking the teenager away and telling Mr Smyth to go home.
Some time between this incident and him being found unconscious by the dog-walker at 5.30am, Mr Smyth's path crossed with Wilson on a footbridge.
Wilson, McKinney and a female friend had been making their way home from a party when Wilson was confronted by Mr Smyth, who provoked a physical altercation.
As Mr Smyth and Wilson became embroiled in a fight, the female urged McKinney to try and break the fight up.
However, McKinney joined in the attack, and after Mr Smyth was put to the ground, McKinney ceased the attack but Wilson "would not desist" and continued to attack the victim as he lay on the ground, offering no resistance or defence to the continued assault.
Wilson also admitted robbing Mr Smyth of a cigarette tin.
Wilson, McKinney and the female friend then left Mr Smyth, prompting Mr Justice Weir to say that after Mr Smyth was rendered motionless, none of them made an effort to summon help.
The judge said that due to the "catastrophic nature" of Mr Smyth's head injuries, it would be impossible to tell whether this would have helped or not, but had they raised help it would have "demonstrated some compassion".
Mr Justice Weir did, however, praise the "bravery" of the female friend, who after hearing about Mr Smyth's death two years after the incident, went to the PSNI which in turn led to the arrests of Wilson and McKinney.
He also acknowledged that Mr Smyth was "in an angry and aggressive mood that night".
The manslaughter pleas were accepted by the Crown on the grounds that due to medical evidence including a post-mortem, it could not be determined whether Mr Smyth had sustained the serious head injuries when he was standing, whilst he was on the ground or if he struck his head when he fell on hard ground during the fight.
Defence barrister for Wilson, Arthur Harvey QC, described his client as a hard-working, popular young man but spoke of his "dual personality" that exists when he drinks alcohol, which he said had "led him down every avenue and path that has diminished his life".
Paul Ramsey QC, acting for McKinney, said his client's involvement was "completely out of character".
The barrister also spoke of "a lot of drink involved", saying that McKinney was unaware of the seriousness of Mr Smyth's injuries.