A municipal court in the seaside resort of Malibu, northwest of Los Angeles, rejected pleas by Downey's celebrity lawyer, Robert Shapiro, to give him one more chance with the drug rehabilitation programmes that have been a feature of the actor's life since he was first arrested in June 1996.
"I don't believe your client is committed to not using drugs. There is some level of choice," Judge Lawrence Mira said in passing sentence.
Downey, 34, was a member of Hollywood's "Brat Pack" in the early Eighties and his performance in the title role of Richard Attenborough's Chaplin earned him an Oscar nomination seven years ago. He has been arrested repeatedly on charges relating to possession of heroin, cocaine and illegal weapons and has been through - and failed - no fewer than seven court-supervised drug treatment programmes and admits to being an addict with serious problems.
Appearing in an orange prison jumpsuit yesterday, Downey said: "It is like I've got a shotgun in my mouth with my finger on the trigger, and I like the taste of gunmetal." Despite the relatively heavy sentence, he thanked the judge for his decision and his patience.
Mr Downey's problems with addiction and the law have not stopped him working, although he has never matched the accolades that greeted his performance as Charlie Chaplin.
Favouring roles as people living close to the edge, he was in Natural Born Killers, Oliver Stone's notorious, fast-paced thriller and in Mike Figgis's bittersweet romance One Night Stand, in which he played a man dying of Aids.
The son of a relatively obscure Hollywood film director, Downey has admitted having drug problems from the age of eight. He dropped out of school and flitted between Los Angeles and New York, doing odd jobs before his acting career first took off.
Since his legal troubles began, he has served 201 days behind bars, which will be deducted from his sentence. With further reductions possible for good behaviour, he could be out of prison in anything from one to two years.