Seifert, 44, will also have to write a written code of conduct for the advertising industry as part of her sentence.
Thomas Early, another former Ogilvy & Mather executive at the centre of the scandal, was sentenced to 14 months in prison and fined $10,000 on Wednesday.
Prosecutors sought a tougher sentence for Seifert, because as executive group director at Ogilvy New York, she was in charge of the Office of National Drug Policy account.
Seifert, who was sobbing when the sentence was read, issued a statement following the hearing.
"The last of human freedoms is the ability to choose one's attitude in the face of adversity. I hope I will have the strength to handle what lies ahead with courage and grace," she said.
Seifert was president of Omnicom's TBWA\Chiat\Day New York but resigned her job a week after being found guilty of fraud and conspiracy charges in February this year.
Her sentence will begin on September 6. As well as serving 18 months in prison, she will serve a further two years under supervision, as will former Ogilvy finance director Early.
Both pleaded innocent to charges of filing false claims, making false statements and conspiring to defraud the government.
There is a chance the US Immigration authorities may deport Seifert, a British citizen, back to the UK to finish her sentence.
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