William Balfour was on trial for the 2008 murder of Jennifer's mother Darnell Hudson Donerson, 57; her brother Jason Hudson, 29; and nephew Julian King, 7.
Sources in the Chicago courtroom tell RadarOnline.com that the Dreamgirls actress arrived just 10 minutes before the verdict was to be read and sat with her fiance and sister.
William was found guilty of 1st degree murder on all counts, home invasion and residential burglary.
Eyewitnesses say Jennifer broke out in tears when the verdict was read.
Jennifer was one of the first to testify in the trial and since then she spent every day quietly sitting in the courtroom on a fourth-row bench watching the legal wrangling unfold.