Mapes was charged with one count of making false declarations before a grand jury and one count of attempted obstruction of justice.
The 68 year old was found guilty on both counts after roughly five hours of deliberating.
Legal analysis of Tim Mapes verdict
"Whenever there is a quick verdict, it usually is bad for the defendant. This is not always the case, especially in a case that it seems is pretty clear the defense is winning. But on a case, kind of hope is lost early on if the verdict comes back as quickly as this did. So I would imagine the defense team's instincts clearly were right,"ABC7 legal contributor Tony Thedford said.
Mapes did not react while the verdict was read and did not speak with reporters on his way out of the courthouse.
He's the second in his circle to fall to federal charges.
Mapes was accused of trying to protect former Illinois House Speaker Madigan when he was his chief of staff.
Thedford said the case against him was pretty cut and dry.
"So the issue for the jury was rather simple. Whether or not they believed that he was telling the truth, when he said he did remember specific things. And unfortunately here, those things that he didn't remember, seemed to coalesce around this relationship between the claim and former Speaker Madigan and appeared suspect," he said.
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For the decades he served as Madigan's chief of staff, Mapes was known in Springfield to be very meticulous and dedicated to details.
Yet, prosecutors said when it came to answering basic questions during a grand jury proceeding about Madigan's longtime relationship with the former House speaker's trusted advisor, Mike McClain, Mapes lied over and over.
In her closing argument, Assistant U.S. Attorney Julia Schwartz told jurors, "Mr. Mapes was no dummy, he knew exactly what was being asked of him, and he tried to shut down any further questioning by pretending he didn't remember, and that was a lie."
Mapes was given full immunity by prosecutors during their investigation into a bribery scheme involving Madigan and McClain.
Schwartz said, "Mr. Mapes could have been a star witness for the government."
SEE ALSO: Tim Mapes does not testify as defense wraps up for Madigan confidant; prosecution rebuttal tomorrow
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Instead, "he decided to stay in the fox hole and protect the boss, something he has done for 25 years."
Presenting 15 witnesses and dozens of wiretapped phone conversations over a nearly two-week trial, prosecutors tried to show the very close relationship among Mapes, McClain and Madigan.
The government said Mapes brazenly lied to the grand jury when asked several similar questions about the nature of Madigan and McClain's relationship.
In his closing argument, defense attorney Andrew Porter told jurors, Mapes "couldn't remember what he didn't know."
Porter said during the grand jury proceedings, Mapes was "not trying to protect the boss; he was trying to answer honesty and carefully."
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Describing Madigan as a very private person who prefers one-on-one conversations, Porter argued, "Tim Mapes didn't know what Madigan and McClain were saying in their private conversations."
Porter said Mapes is charged with perjury over not recalling insignificant past legal events that had nothing to do with criminal activity involving Madigan and McClain.
His conviction, and that of his former colleague Michael McLean, are considered pieces chipped away at Madigan's guardsmen during his longtime hold on the state Capitol.
This conviction is welcome news to state republicans.
"Certainly it's a good day that justice is being served and is long overdue for the state of Illinois, but it's also certainly a sad day because here we have jurors and the federal court stepping up to do what should have been done a long time (ago)," Deputy House Republican Leader Ryan Spain said.
"Today's guilty verdict is the latest in a string of examples of the urgent need for ethics reform to clean up corruption in Illinois," said state Rep. Patrick Windhorst, R-Metropolis.
The verdict did come as a surprise to Chicago Democrat state Rep. Kelly Cassidy.
"His No. 1 reason for being was to serve the speaker, whether it was cruelty to his staff or an attempt to exercise absolute control or his belief he was immune; you don't lie to the feds," Cassidy said.
In a statement late Thursday afternoon, Acting U.S. Attorney for the Northern District of Illinois Morris Pasqual said Mapes' conviction "should stand as a clear message to witnesses" who are called to appear in front of a grand jury.
"Perjury and obstruction of justice are serious offenses that strike at the heart of the truth-seeking mission of the grand jury," Pasqual said.
Members of the jury also declined to speak with reporters as they left the courthouse in small groups Thursday afternoon.
The obstruction of justice charge carries a maximum of 20 years in prison.
Mapes' sentencing date is tentatively set for 9:30 a.m. Jan. 10.
Capitol News Illinois contributed to this report.