The impact of the verdict on this year's season for Northug was unclear.
The skier won four Olympic medals – two golds, one silver and one bronze – in the 2010 Winter Games in Vancouver, but returned emptyhanded from Sochi early this year.
The Trondheim court ordered Northug, who initially tried to blame the crash on a friend who was in the car, to pay a 185,000 kroner ($28,844) fine, corresponding to one and a half month's salary.
It also slapped him with a lifelong driving ban, which in practice usually means a five-year suspension.
The 28-year-old athlete had opted for a speedy trial by pleading guilty and accepted the ruling immediately.
His options could include asking for the sentence to be postponed or to wear an electronic ankle bracelet.
"This episode will haunt me and I will be ashamed of it all my life," he said in a brief speech to the press.
"Once again I apologise… I'm prepared to take my punishment," he said.
The incident took place in the early hours of May 4, when Northug's Audi A7 was found totalled after crossing a roundabout and crashing into a guard rail.
The athlete fled the scene on foot and was arrested in his home after leaving a badly injured passenger behind.
Police say the car was driving at a speed of at least 83 kilometres (52 miles) per hour in a 40 kilometre per hour zone, and that Northug one hour later had a blood alcohol level of 1.65 grammes, well over the 0.2 gramme legal limit.
The Olympian had faced six charges: driving under the influence, reckless driving, speeding, failure to provide assistance, failure to stop at the scene of an accident and making a false accusation.
In his first statements to the police Northug said three times that his friend had been driving the vehicle, a claim he later retracted and attributed to still being drunk.
The court found that Northug's alcohol intake was a mitigating circumstance with regards to the false accusation charge. The prosecution had asked for a 60-day prison sentence, a 200,000 kroner fine and a lifetime driving ban.