Jack Skeen, the district attorney in the town of Tyler in eastern Texas, said the defendant, Kenneth Payne III, was a habitual offender with ten previous convictions.
Payne's record includes convictions for stealing tools and a bag of sweets.
"The message from this case is that regardless of the value of the item stolen, a habitual criminal reaches a point where a jury and a prosecutor say enough is enough," Mr Skeen told the newspaper.
"The jury refused to put him back out on the street [although] this theft did not involve a significant dollar amount."
Worldwide publicity
He said he was publicly defending the sentence, handed down by a jury late last month, because he had been getting calls from journalists from around the world since the case was reported this week.
The district attorney said Payne had been offered a plea bargain that included an eight-year sentence, but had rejected the offer.
Shoplifting carries a maximum $500 fine in the Texas, whereas habitual offender status raises the charge to felony theft with a maximum sentence of up to 20 years in prison.
Assistant district attorney Jodi Brown, prosecuting in the trial, had asked the jury for 16 years and the jury agreed.
She said on Monday that she was a little surprised by the severity of the sentence.
"But it was a king-size," she added. "And it was a Snickers bar. If it was a Milky Way, we probably wouldn't have even tried him on it.
"He was no stranger to the system and disregarding the law."
Payne's lawyer has said he will appeal.