McMillen, 31, went on trial Monday before U.S. District Judge Sharion Aycock in Aberdeen.
His jury was out 65 minutes Tuesday before returning a guilty verdict. He was placed in custody of the U.S. Marshal Service.
Aycock will sentence him after a report by the U.S. Probation Service.
A March 2010 federal grand jury accused McMillen of possessing computers and storage files with images of child pornography that were shipped across state lines.
Maximum penalty is 10 years in prison and a $250,000 fine.