Leeds, 53 places higher on the league ladder than Newport, led 1-0 for most of a pulsating third-round tie at Rodney Parade.
But a Conor Shaugnessy own goal and substitute Shawn McCoulsky’s towering header put County in the fourth round for the first time since the 1978-79 season.
A grim afternoon for Leeds got even worse when Samuel Saiz was sent off in stoppage time, the Spaniard banished after allegedly spitting at County midfielder Robbie Willmott.
Christiansen made nine changes from the New Year’s Day draw at Nottingham Forest, but denied he had taken either the competition or the opposition lightly.
“If you look, every player we had in the first XI today is from the first team or has possibilities in the first XI,” insisted Christiansen.
“For this we expect more and it should be enough to pass to the next round. Against Burnley (in the Carabao Cup) I also made 10 changes and we had the perfect performance.
“It was the same against Leicester, we also made nine or 10 changes and did very well. So why not today? Why is it different? They know what the FA Cup means and I know it.”
On the Saiz spitting allegation, Christiansen added: “I do not know, but if that’s right why he got the red card then it’s a problem. But I believe he didn’t do it.”