Seale, 72, was convicted in June on federal charges of kidnapping and conspiracy in the deaths of Charles Moore and Henry Hezekiah Dee, two 19-year-olds who disappeared from Franklin County on May 2, 1964.
The young men's bodies were found two months later in the Mississippi River.
Seale showed no emotion as the verdict was read by U.S. District Court Judge Henry Wingate. Wingate added that the crimes committed 43 years ago were "horrific" and "justice itself is ageless." He denied a defense motion to allow Seale to be free on bond while his case is appealed. Wingate agreed to assign Seale to a prison where his health needs can be met. He has cancer, bone spurs and other health problems.
The families of the two victims stood about 10 feet from Seale, but he never made eye contact with them.
The prosecution's star witness against Seale was Charles Marcus Edwards, a confessed Klansman who received immunity from prosecution for his admitted role in the abductions and his testimony.
He testified that Seale and other Klansmen abducted Dee and Moore near Meadville, forced them into the trunk of Seale's Volkswagen and drove them to a farm. The two were later tied up and driven across the Mississippi River into Louisiana.
Edwards said Seale told him that heavy weights were attached to the teenagers and they were then dumped alive into the river.
Seale was arrested on a state murder charge in 1964, but the charge was later dropped. Federal prosecutors say the state charges were dropped because local law enforcement officers in 1964 were in collusion with the Klan.