"In God We Trust" has been used on coins since 1864, but it was almost 100 years before it became the official motto. On July 30, 1956, the 84th Congress passed a law and President Eisenhower signed it into law deciding that "In God We Trust" would become the nation's motto. In 1957, "In God We Trust" began appearing on paper money as well. On January 26, 2011, the House of Representatives with the Senate in agreement, reaffirmed the phrase as the national motto. Read the full history of the motto in this Congressional report.
Source: congress.gov