The sight of North Korean defector Ji Seong-ho hoisting a pair of homemade crutches above his head during extended applause will be one of the enduring images of the 2018 State of the Union address.

The 35-year-old managed to escape the dictatorship in 2006, President Donald Trump explained, even though he’d lost a leg and a hand stealing coal from a train as a teenager.

Since then he has worked as an activist helping others escape and directing pro-democracy radio broadcasts into North Korea. That won him the attention of the Oslo Freedom Forum—a human-rights group. He spoke to The Wall Street Journal on Jan. 23.

The story he told began in North Hamgyong province, a frigid northeastern region of North Korea near its border with China and Russia, at the height of a famine in the 1990s. His town was near Camp 22, a notorious gulag 30 miles long and 20 miles wide where tens of thousands of prisoners toil at mines, farms and other hard labor. Trains laden with coal rumbled regularly out of the camp.

“The rumor we heard was if the prisoners failed to mine their daily quota of coal, they weren’t allowed to leave the mine,” said Mr. Ji.