When was in God we trust added to the five dollar bill?

The motto was added to $5 bills beginning with the 1963 series. It had been added to $1 bills in 1957.

Congress passed a law in 1955, at the height (or depth, depending on your views) of Cold War tensions with the Soviet Union, mandating the phrase on US paper currency. It was seen as a way of distinguishing the US from the "godless communists" who at that time threatened western democracies.

The motto first appeared on 2¢ coins in 1864 (during the Civil War) and was gradually added to all denominations after that time. However, it was never put on paper money until the 1955 law was written.