Williams was born in Haverfordwest, Pembrokeshire. His father, a primary school teacher from Pembrokeshire, spoke Welsh and his mother spoke English. In his early years he himself spoke only English.

In 1911 his father was appointed headteacher of the primary school at Mynachlog-ddu, Pembrokeshire and it was there that Waldo Williams learnt to speak Welsh. In 1915 Williams's father moved to be headteacher of Brynconin School, the primary school at Llandysilio, Pembrokeshire.

After attending the Grammar School at Narberth, Pembrokeshire he studied at the University College of Wales, Aberystwyth where he graduated in English in 1926. Afterwards he trained as a teacher and worked in various schools in Pembrokeshire, the rest of Wales and England. He also taught night classes organised by the Department of Extre-Mural Studies, University College of Wales, Aberystwyth.

Williams married in 1941, but his wife died in 1943, and he did not remarry.

During the Korean War he refused to pay his income tax on pacifist grounds. He continued his protest until the ending of compulsory military service in 1963. During his campaigning he was sent to prison.

He stood as a parliamentary candidate for Plaid Cymru in the Pembrokeshire constituency at the 1959 General Election when he won 4.32% (2.253) of the votes.

He died in 1971 at St. Thomas's Hospital, Haverfordwest, Pembrokeshire and is buried at Blaenconin Chapel burial ground in Llandysilio.

Williams's poetry shows many influences from William Wordsworth and Walt Whitman to Welsh hymns and the strict alliterative metres of traditional Welsh poetry, known as cynghanedd.

Williams belongs, first of all, to the Welsh tradition of the bardd gwlad (=folk poet), a poet who served his locality by celebrating its life and people in verse.

But he was also a poet inspired by the mystic revelation he had as a youth about the unity of the whole of humankind. This revelation was realised in the cooperative and harmonious living he witnessed amongst the farming communities in the Preseli Hills and reflected in feelings of belonging, knowing, and a desire that people should live together in peace — constant themes in his poetry. It inspired some of his greatest poetry, including Mewn dau gae (=In two fields) (1956), perhaps his greatest poem of all, which celebrates the very moment of this revelation.

Other well-known poems by Williams include Cofio (=Remembering) (before 1936), Y tangnefeddwyr (=The peacemakers) (1941), Preseli (1946), and Pa beth yw dyn? (=What is it to be human?) (1952).

• 1911 - Moves to Mynachlog-ddu when his father becomes headteacher of primary school.
• 1915 - Moves to Llandysilio, Pembrokeshire when his father is appointed headteacher of primary school.
• 1917 - Attends grammar school at Narberth.
• 1923 - Begins study at University College of Wales, Aberystwyth.
• 1926 - Graduates in English and trains as a teacher.
• 1928 - Begins to teach in at various primary schools in Pembrokeshire.
• 1936 - Publication of Cerddi'r plant (=Poems for children)
• 1941 - Marries Linda Llewellyn.
• 1942 - Moves from Pembrokeshire to teach in north-west Wales.
• 1943 - Linda Llewellyn dies.
• 1944 - Moves to teach at Kimbolton, Huntingdonshire.
• 1946 - Moves to Lyneham, Wiltshire.
• 1949 - Returns to Wales.
• 1950 - The Korean War; resigns from teaching to begin protest of non-payment of income tax against the war. Protest continues after the war until the end of compulsory military service in 1963.
• 1953 - Leaves the Baptist church to join the Religious Society of Friends, commonly known as Quakers.
• 1956 - Publication of Dail pren (=The leaves of the tree).
• 1959 - Stands as the Plaid Cymru candidate in the Pembrokeshire constituency at the General Election.
• 1960 - Imprisoned for six weeks for non-payment of income tax.
• 1961 - Imprisoned for a further period for non-payment of income tax.
• 1963 - Resumes teaching at various primary schools in Pembrokeshire.

Waldo Williams memorial, Rhos-fach, Mynachlog-ddu.

Dail pren (=The leaves of the tree), 1956, was the only volume of poetry published by Williams during his lifetime. A new edition was published in 1991 by Gwasg Gomer, ISBN.

Cerddi Waldo Williams (=The poems of Waldo Williams), 1992, a selection of his poetry edited by J. E. Caerwyn Williams.

Waldo Williams: rhyddiaith (=Waldo Williams: prose), 2001, a selection of his prose writings edited by Damian Walford Davies. Includes writing in both Welsh and English.

Cerddi'r plant (=Poems for children), 1936, a volume of poetry including the work of Williams and E. Llwyd Williams.

The old farmhouse, 1961, Williams's translation of Yr hen dy ffarm by D. J. Williams (1953).

A significant collection of his poems have been translated into English by Tony Conran. See References.