James Meyer, 53, of Salisbury, Connecticut, was also ordered to pay $13 million in restitution and to forfeit $3.9 million.
U.S. District Judge J. Paul Oetken said he believed Meyer was genuinely remorseful and was primarily a "kind, caring, thoughtful" man who committed a serious offence, selling unauthorized artworks on at least three occasions from 2006 to 2011, pocketing more than $4 million.
Oetken said Meyer's victims included Johns and those who purchased the art only to learn they were unauthorized. The judge said the fall of a good man was a familiar mystery.
"Why do such good people end up engaged in these sorts of crimes?" the judge asked. He noted that defendants like Meyer usually are remorseful once caught, leading him to wonder whether remorse stems more from having been caught than from the crime itself.
Meyer, Johns' studio assistant for over 25 years, apologized and expressed regret at his severed relationship with the artist.
"I am truly devastated that I destroyed the close relationship that I had with the man who was my mentor, employer and friend since I was 21 years old. I took for granted and betrayed someone who will forever have great meaning in my life. For that, I have profound remorse," he said in a packed courtroom.
When he pleaded guilty to interstate transportation of stolen property in August, Meyer told the judge he removed artwork from Johns' studio in Sharon without the artist's permission so he could sell them at a gallery.
In a presentence submission to the court, defence attorney Morris Fodeman asked for leniency, saying a Manhattan gallery owner persuaded Meyer over a three-year period to give up some of Johns' works. He said the gallery owner eventually sold 37 works of art for $9.98 million.
Assistant U.S. Attorney Daniel Tehrani said in a presentence submission that a significant sentence was necessary "to protect the integrity of the art market and the livelihood and artistic control of those who contribute to it."
The prosecutor said one buyer who agreed to return the artworks to Johns had spent over $7 million on them.
Tehrani noted that Meyer tried to cover up the scam by requiring buyers of the art agree not to resell, exhibit or loan the art for eight years. In return, he gave the buyer fictitious authentication papers.
Tehrani said Meyer returned 41 artworks after his arrest, nearly half of the 83 works he had stolen or unlawfully maintained.
Prosecutors said part of Meyer's responsibilities as Johns' assistant was to destroy art Johns was not satisfied with and to protect a studio file drawer containing unfinished pieces of art by Johns.Suggest a correction