Michael McWhorter and Joe Morris said that when they arrived at the Dar al-Farooq Islamic Center in Bloomington on August 5, 2017, they broke a window and threw a lit pipe bomb and a petrol mixture inside, causing an explosion, fire and extensive damage.
McWhorter, 29, and Morris, 23, of Clarence, Illinois, pleaded guilty on Thursday to five counts in connection with the mosque attack, the attempted bombing of an Illinois abortion clinic, armed robberies and other crimes.
A third defendant, 47-year-old Michael Hari, who prosecutors said directed the bombing, remains in federal custody in Illinois.
The plea agreements portray Hari as the ringleader of a militia group called the White Rabbits, which included Hari, McWhorter, Morris and at least five other people. Hari's trial is set for July.
The guilty pleas of McWhorter and Morris came a day before three members of another militia were set to be sentenced for a foiled plot to massacre Muslims in southwest Kansas by blowing up a mosque and apartments housing Somali immigrants.
That attack, planned for the day after the November 2016 presidential election, was thwarted after another member of the group tipped off authorities.
Anti-Muslim attacks have spiked since Donald Trump became the US president, rights groups say.
The Council on American Islamic Relations (CAIR) recorded 300 hate crimes in the US targeting Muslims in 2017. That number marked a 15 percent increase from 2016, when CAIR recorded 260 incidents targeting Muslims.