After receiving his doctorate, he passed his bar exam of the canton of Aargau and was a law clerk at the District Court of Baden in 1900, as well as an editor of the Aargauer Tagblatt.

In 1902, Käslin joined the federal administration and was until 1911 adjunct in the Department of Justice and Police.

After the unexpected death of Chancellor Adolf von Steiger, Käslin was elected by the Federal Assembly unopposed.

Since the Catholic Conservative People's Party had a second Federal Council seat since 1919, they renounced this in a counter-bid.

In the area of public affairs Käslin is considered a pioneer of public relations, as he organized the first head of the Federal Chancellery regular meeting with the press.