He was a member of the collaborationist government Nasjonal Samling in occupied Norway during World War II and a Standartenführer (Colonel) in the Schutzstaffel.

Riisnæs was the public prosecutor in the famous case against Per Imerslund and other Norwegian Nazis who had broken into the home where Leon Trotsky had been staying before his deportation from Norway to Mexico.

As Minister of Justice, Riisnæs was responsible for changing the Norwegian legal system to legitimise the Nazi actions, and authorized the persecution of those who would not cooperate with the German occupiers.

[3][4] After collapse of the Quisling government at the end of the war, Jonas Lie, Henrik Rogstad and Riisnæs retreated to an NS gathering place outside of Oslo.

In 1974 he emigrated to Sicily, Italy, and later to Vienna, but returned to Oslo in 1985, where he lived for three years in a nursing home until his death.