Pae Jun-ho, known in the US as Kenneth Bae, smuggled anti-government materials and plotted the state's downfall, a Supreme Court spokesman told state news agency KCNA.
He denied Mr Bae was being used as a "political bargaining chip" with the US.
Mr Bae was detained last year after entering North Korea as a tourist.
North Korea has arrested several US citizens in recent years, including journalists and Christians accused of proselytism.
They were released after intervention from high-profile American figures, including former Presidents Bill Clinton and Jimmy Carter, both of whom went to Pyongyang.
The court spokesman told KCNA that Mr Bae was "dispatched to China as a missionary".
There he was involved in setting up "plot-breeding bases" from which he "infiltrated at least 250 students" into the North Korean port city of Rason with the aim of "bringing the government down".
US citizens held by N Korea
"He committed such hostile acts as egging on overseas [North Korean] citizens and foreigners to perpetrate hostile acts to bring down its government while conducting a malignant smear campaign," he said.
He added Mr Bae's full confession was the reason why he did not receive a possible death sentence.
"He dared commit such hideous crimes as hurting the dignity of the supreme leadership," he said.
The spokesman denied that Mr Bae was "not tried in a transparent matter and [the North] was trying to use this issue as a political bargaining chip".
Mr Bae, 44, was arrested in November as he entered Rason, a special economic zone near North Korea's border with China.
South Korea activists believe he was arrested for taking pictures of starving children. Other reports have described him as a devout Christian.
His conviction on 30 April came amid high tension between the US and North Korea, after Pyongyang's third nuclear test.
The UN expanded sanctions against the communist state in March, in the wake of its 12 February nuclear test and December long-range rocket launch.
Pyongyang reacted angrily both to the measures and the annual US-South Korea military exercises, threatening to attack US military bases and cutting key hotlines with its southern neighbour.

Vaughan joined Blues permanently from Huddersfield in the summer after a loan spell last season but has featured just once this term.
The 28-year-old former Everton, Derby, Leicester and Norwich striker is the Premier League's youngest ever scorer at 16 years and 271 days.
Vaughan has signed a two-year deal and can play against Walsall on Saturday.
Find all the latest football transfers on our dedicated page.

The children's laureate - always a well-known writer or illustrator of children's books - is chosen by a panel of judges from the book world. It comes with a bursary of Â£15,000, not to mention a shiny, silver medal.
The role has a pretty loose job description - the laureate has to "promote and encourage children's interest in books, reading and writing".
Jacqueline Wilson's "obsession" while she was children's laureate was to "get everyone to read aloud to children, all children, from the age of nought to 11".
Malorie Blackman revealed that she spoke to over 20,000 teenagers when she was in the role but arguably her main achievement was setting up the UK's Young Adult Literature Convention, which now takes place each year as part of the London Film and Comic Con.
Chris Riddell has spent much of his two years defending school libraries and librarians, and promoting the art of illustration at live events. In his final speech before he passes on the baton, he said the lack of investment in school libraries is a "blight on the intellectual development and creative future of all our children", as well as highlighting the issues faced by child refugees in the UK.
Quentin Blake, the first children's laureate and best-known for his collaborations with Roald Dahl, produced a book called A Sailing Boat in the Sky in collaboration with 1800 French-speaking schoolchildren.
Follow us on Facebook, on Twitter @BBCNewsEnts, or on Instagram at bbcnewsents. If you have a story suggestion email entertainment.news@bbc.co.uk.
North Korea has released details of the alleged crimes of a US man it sentenced to 15 years of hard labour.
[NEXT_CONCEPT]
Bury have signed striker James Vaughan on a free transfer after he left Birmingham City by mutual consent.
[NEXT_CONCEPT]
Charlie and Lola creator Lauren Child has been named as the new children's laureate, taking over from Goth Girl author Chris Riddell.