Ismail, alias Muhammad Ikhwan, was found guilty of terrorism and illegal possession of explosives used in the August 2003 bombing of the Marriott hotel in which 12 people were killed.
Judge I Wayan Rena Wardana told the South Jakarta district court on Thursday that the defendant was "legally and convincingly proven to have engaged in a crime of terrorism", and had taken part in a crime causing "widespread terror".
Previously the highest sentence handed down in connection with the Marriott attack had been 10 years.
The bombing has been blamed on the Jemaah Islamiya group, which is also held reponsible for the October 2002 Bali nightclub blasts in which 202 died. The group denies responsibility.
Ismail, 24, demanded time to consider an appeal but said he was unfazed by the sentence.
"What is clear is that this verdict is only made by humans, and I do not fear a verdict issued by humans," he said when asked by judges for a response.
Prosecutors said Ismail, who has previously expressed regret for the attack, had taken part in preparing and carrying out the bombing under orders from two Malaysian bomb experts - Azahari Husin and Nurdin Mohammed Top.
"What is clear is that
this verdict is only made by humans, and I do not fear a verdict issued
Ismail, alias Muhammad Ikhwan
Both Nurdin and Azahari have been named as prime suspects in the 9 September embassy attack and are the subject of a huge police manhunt.
Meanwhile, Indonesian police have said they have arrested two people suspected of involvement in last week's deadly bomb attack on the Australian embassy in Jakarta.
Police confirmed the couple, arrested late on Wednesday near the port city of Surabaya on central Java island, were being held on "suspicion" of being linked to the 9 September bombing in which nine died and 180 were hurt.
Provincial police spokesman Endro Wardoyo said the two were seized in a raid on a house in the town of Sidoarjo during what appeared to be an initiation procedure. Three others managed to escape.
He said the two arrested were Indonesian citizens, but did not release their names. However, local media identified them as Rahmat Allah and Farida.
Wardoyo said during the raid the suspects had attempted to dispose of some documents in a well but police managed to recover the evidence, which was only slightly damaged.
The Marriot hotel bombing killed
12 people in Jakarta last year
The pair were "part of a group of radicals in East Java that we've been after for some time", said national police spokesman Sunarko Danu Ardanto.
But he declined to say whether they were members of the Jemaah Islamiya group.