Born in Birkenhead, England (near Liverpool) to Welsh parents, Davies was raised a Methodist. After working briefly as a clerk for a shipping company, he moved to London as secretary to a strike leader who had been elected Labour MP. In London, he met George Bernard Shaw, who urged him to go into politics; instead, he chose to attend Richmond Theological College, a Methodist seminary affiliated with the University of London, and to join the Methodist ministry. While studying, he met Muriel Hannah (1906-2009), the daughter of his family's Methodist minister, and they were married in 1927.
Davies served as minister at the Becontree Methodist Central Hall in Illford, a London suburb, 1925–28, before moving to America. He headed three churches in Maine: in Goodwin's Mills and Clark's Mills, (commuting to Boston University for classes), and, from 1929, the Pine Street Methodist Church in Portland. In Portland, he worked as a used car salesman for a week in order to investigate the question of whether or not it was possible to be a businessman and adhere to "Christian" honesty and integrity; he then wrote a strong critique of prevailing business practices.
Also in Portland, Davies was exposed to Unitarian ideas by Vincent Silliman, minister at the First Parish Unitarian Church there. In 1933 Davies became a Unitarian, joining the American Unitarian Association (AUA), and became minister of the Community Church of Summit, New Jersey which had allied itself with the Community Church movement. He reaffiliated the church to the Unitarian denomination, with the new name Unitarian Church in Summit, and at the same time calling for an opening up of the denomination.
While preferring non-violence, he moved his position from appeasement of Germany in 1938 to interventionist in 1939. He thenceforth advocated Unitarian activism in world questions, and became a leader in the reform movement, Unitarian Advance. His first book was American Destiny (1942), in which he advanced the idea that American "faith in [individual] freedom is the only faith which can unite the world." Citing the Boston minister, William Ellery Channing  (1780-1842), who had called Unitarianism the "universal church," Davies became increasingly active in denominational affairs. He joined the Metro New York Council of Churches, where he met and befriended the Unitarian activist minister John Haynes Holmes  (1879-1964), and became acquainted with Margaret Sanger (1879-1966), the founder of Planned Parenthood.
In 1943 Davies was appointed minister of All Souls Church, Unitarian in Washington, D.C., a church which he led to national prominence through his activist and principled ethical stands. He advocated the Atomic Energy Act of 1946 and, while generally against Communism, decried the methods and hearings of the House Un-American Activities Committee (HUAC). He championed integration in Washington and elsewhere. He actively spearheaded the creation of Unitarian churches in the area, organizing in 1950 what is now known as the Greater Washington Association of Unitarian Universalist Churches (GWA)  and serving as its Chair, 1950-57.
Davies used his pulpit to champion liberal causes and was a friend and confidant to some of Washington's most powerful people. Justice Hugo Black considered him a very good friend and it is fair to say that his ideas were influential in liberal political circles.
Davies received an honorary Doctor of Divinity degree from the Meadville Lombard Theological School in 1947 and an honorary Doctor of Humanities degree from Howard University in 1955. Following his death, some of his papers were bequeathed to Harvard Divinity School, however, the majority of Davies' papers reside at Meadville Lombard Theological School."The religions of the creeds are obsolescent... the basis of their claims expired with yesterday."
"It (liberalism) causes us to put our trust in the free exertions of our own minds instead of in the dogmas of the long established churches."
"This ancient God of miracles and interventions... is really dead. There is no longer any kindness in letting anyone cling to such a fantasy. For if that is where we put our faith, our dependence, or reliance, we shall be wiped off the face of the earth."
"There is no God in the sky. God is in the heart that loves the sky's blueness. There is no army of angels, no hosts of seraphim and no celestial hierarchy. All this is man's imaginings."