Wilson was capped by Ireland against Japan in 2005, while Mordt has featured for the England Saxons side.
Wilson said: "Northampton are an ambitious club. They are on their way up and I want to be a part of that.
"I want to improve as a player and I have always wanted to play in England.
"It is a physical league and the quality has risen dramatically. I hope I can fit in to how the game is played there.
"I have been at Ulster so long, it will be different. But it's also a positive move for me."
Northampton, relegated last season, are currently running away with National League One.
They have won all 19 of their games and currently hold a 10-point lead over closest challengers Exeter.