For more than 20 years, tech tycoon William H. Millard was one of the world's most elusive tax exiles, leaving financial footprints in Singapore, Ireland and other locales while racking up an unpaid tax bill of more than $100 million.

The 79-year-old founder of the ComputerLand Corp. retail chain was last seen by tax authorities on the remote Pacific Island of Saipan, where he lived, in August 1990. A few years after selling his company, the man once listed as one of the richest people in America suddenly vanished.

Until...