The Manhattan federal jury was in its 13th day of deliberations in the case against Stewart, 65, a fixture on the New York legal scene for 30 years.
"Stewart is a legend in certain legal circles - a ferocious defender of her clients, some of whom were the most unpopular in recent memory," said CBS News Legal Analyst Andrew Cohen. "But clearly the feds thought she went too far for this client..."
Stewart faces up to 20 years in prison giving material support to terrorists and defrauding the U.S. government.
The most serious counts on which she was convicted were conspiracy and providing and concealing material support of terrorism.
"Today's verdict is an important step in the Justice Department's war on terrorism," said Attorney General Alberto R. Gonzales in a statement. "The convictions handed down by a federal jury in New York today send a clear, unmistakable message that this Department will pursue both those who carry out acts of terrorism and those who assist them with their murderous goals."
Stewart sat stoically in a courtroom filled with her supporters, who gasped when the verdict was read.
The anonymous jury also convicted a U.S. postal worker, Ahmed Abdel Sattar, of conspiracy for plotting to "kill and kidnap persons in a foreign country," for instance by publishing a fatwah urging the killing of Jewish people and their supporters "wherever they are."
A third defendant, Arabic interpreter Mohamed Yousry, was convicted of providing material support to terrorists.
The trial focused attention on the boundaries between zealous advocacy and alleged criminal behavior by a lawyer.
Some defense lawyers saw it as a government warning for attorneys to tread softly in terrorism cases. CBS's Cohen pins this as a recent shift in government oversight of this part of the legal system.
"The government for years now has gotten permission to monitor certain conversations between attorney and client, conversations that a decade ago would have been sacrosanct," Cohen said. "And this verdict is the fruit of that effort.
"Lawyers around the country, especially ones who represent criminal defendants, were watching this case very closely because of what it says about the government's new efforts to involve itself in the relationship between attorney and client.
"The old rules about that relationship clearly have changed," Cohen said.