Joseph Bierbrodt was accused of assaulting Capitol police.
The government shared images from inside the building, allegedly showing him and his brother.
Bierbrodt pleaded guilty before a jury trial could take place. He faces sentencing in February.
He was originally charged with eight federal crimes, including entering a restricted building, and causing physical violence and disorderly conduct in a restricted building.
The FBI said surveillance video shows Bierbrodt's brother breaching a locked fire door, allowing the crowd to enter the Capitol, including Bierbrodt, who allegedly wore a red hat and an American flag face covering.
Prosecutors say he clashed with multiple officers, and was pepper-sprayed and left bloodied before leaving the Capitol with his brother.
The Illinois National Guard said Bierbrodt retired from service in 2018.
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ABC7 Chicago reported on an unrelated story involving Bierbrodt in 2018, when he accompanied the daughter of a fallen soldier to her daddy-daughter dance. Months later he was honored for that gesture at Chicago's Memorial Day weekend remembrance.
Federal prosecutors said they used a screen shot apparently from the ABC7 report to help identify him.
See also | Chicago cop charged in Jan. 6 Capitol attack told FBI he enjoyed 'invading capitols': document
The defendant's brother, William Bierbrodt, was also charged with similar federal crimes.
After identifying the brothers as suspects, law enforcement officials used the images to locate Joseph Bierbrodt in late October 2021 in Sheridan, Illinois. His brother was located at his home in Florida.
See also | Chicago man sentenced to 4 years in prison for role in January 6 riot at US Capitol