The jury selection process was stalled until two months ago because of delays involving funding for the defense. The trial before Superior Court Judge James Bodiford is expected to last the rest of the year.
Bodiford told the jurors at 2 p.m. Wednesday they would hear the case.
Opening statements are scheduled for Monday.
Nichols is charged in the March 11, 2005, shooting spree. He is accused of killing Superior Court Judge Rowland Barnes and court reporter Julie Ann Brandau in the courthouse and sheriff's Deputy Hoyt Teasley outside the courthouse. Nichols also is accused of killing U.S. Customs agent David Wilhelm later that day at a north Atlanta home Wilhelm was renovating.
Prosecutors will ask for the death penalty if Nichols is convicted.
Nichols has pleaded not guilty by reason of insanity.