Seifert was president of TBWA's New York office, which she joined from Ogilvy & Mather where the overbilling on the government's anti-drugs campaign took place.
Her position at TBWA has looked unclear for some time. The agency has since brought in Brett Gosper to take on the role of president.
In a statement, TBWA said that Seifert and the agency had agreed to her resignation, which became effective as of February 28.
Seifert and Early now face sentencing on May 16 and could be looking at up to five years in prison, as well as a fine.
They were found guilty by a jury in New York of 10 charges, including making false claims and conspiracy. The jury deliberated for under six hours and the guilty verdict was unanimous.
The case concerned O&M's work on the US government's Office of National Drug Control Policy in 1999 and 2000.
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