Boe, younger brother of Olympic and three-time world champion Tarjei, was hotly tipped to take the junior men’s 10km sprint title on the opening day in Osrblie but fortune favoured Christiansen.
The 19-year-old made just one error in the shooting range and beat off Russian Alexandr Loginov, who was perfect with the rifle, by just one second to take gold in 23:55.7minutes.
Maxim Tsvetkov was more than 30 seconds back to take bronze in 24:30.1 and join compatriot Loginov on the podium while Boe had to make do with eighth after four penalties in the range.
“Normally Johannes is better than me but he missed a few too many and maybe it helped me that nobody was looking at me,” said Christiansen.
“I started hard because I had decided to be offensive. After the standing, when I missed one, I was told that I led by 20 seconds and I thought I would win.
“But then, somehow, my lead became smaller, I was told it was only seven seconds and then two and I thought I would lose the gold medal but at the finish I won by one second.”
Meanwhile in the junior women’s 7.5km sprint Bulgaria’s Niya Dimitrova shot perfectly to take gold 15.1seconds ahead of Ukraine’s Iryna Varvynets, who was exactly 14 in front of Elisa Gasparin.