Dungannon Crown Court Judge Gemma Loughran told Eamonn Charles Coyle, of Drumlegagh Road South, between Drumquin and Omagh, that from his record "it is sadly clear that despite your age, the use of serious violence is not foreign to you".
However, Judge Loughran said while she found he posed a serious risk of harm to the public, "on balance I have some confidence" he would engage and undertake the work required of him when released back into the community.
The 21-year-old, who has spent a quarter of his young life in prison, will serve at least three years imprisonment before he may be released on the recommendation of the Parole Commissioners, for a further three years, followed by an extended two-year period of supervised licenced parole.
Coyle was unanimously convicted in March of the 11 February 2012 robbery, by a jury prevented of hearing of his 17 previous convictions, including assault and criminal damage to a police car during his arrest in a Campsie Road bar in the early hours of the following morning.
Coyle, originally from Holmview Terrace, Omagh, was just 16 when in April 2009, he broke into the home of his 78-year-old grandfather Francis O'Neill and stabbed him in the neck and strangled him for £80 rent money.
Initially given just a year's detention for the manslaughter, it was later increased to two.
At the time of the robbery Coyle was out on licence only eleven weeks.
However, in October that year he was subsequently returned to Hydebank Young Offenders' Centre to serve the remainder of his manslaughter sentence.
During his three-day trial for the 'Hill Shop' robbery on Omagh's Kevlin Road, the court heard that a baton-wielding Coyle, was one of two men who demanded cash from a young teenage shop assistant punched in the face and chest before Coyle beat him with the baton as he tried to grab cash from the till.
During the commotion the cash was scattered all about, and Coyle and his partner managed to make off with just over £40.
Although wearing a hoodie, Coyle was later identified from CCTV footage as being one of the two attacking robbers.
Judge Loughran said it was a "serious incident which caused some physical injury to the young lone shop assistant and must have been very frightening for him", and that he was "subject to considerable force even when he was complying with your demands".
The robbery, although a relatively unsophisticated, had involved a degree of limited planning, made all the more serious by the fact Coyle was on licence, was with a second assailant and "used gratuitous violence on a vulnerable young shop assistant working alone".
Judge Loughran added later, Coyle had "played a central role and did not plead guilty".
In defence Coyle claimed he had spent almost most of the early evening in the Omagh flat of two friends before going to the Campsie Road bar shortly before 11pm.
Although Coyle agreed with prosecutor, Michael McAleer, that the robbery footage was "horrific", he maintained, "it wasn't me", adding later: "see anyone who thinks I was part of it, they're wrong, they're all wrong."
Despite his claims and the arguments of defence lawyer Ian Turkington that the prosecution case was "utterly bankrupt", and the jury of seven women and five men were being asked "to take a huge leap" of faith, they took just an hour to convict him.