Walcott was born and raised in Castries, Saint Lucia, in the West Indies, the son of Alix (Maarlin) and Warwick Walcott. He had a twin brother, the playwright Roderick Walcott, and a sister, Pamela Walcott. His family is of English, Dutch and African descent, reflecting the complex colonial history of the island that he explores in his poetry. His mother, a teacher, loved the arts and often recited poetry around the house. His father, who painted and wrote poetry, died at the age of 31 from mastoiditis while his wife was pregnant with the twins Derek and Roderick. Walcott's family was part of a minority Methodist community, who felt overshadowed by the dominant Catholic culture of the island established during French colonial rule.
As a young man Walcott trained as a painter, mentored by Harold Simmons, whose life as a professional artist provided an inspiring example for him. Walcott greatly admired Cézanne and Giorgione and sought to learn from them. Walcott's painting was later exhibited at the Anita Shapolsky Gallery in New York City, along with the art of other writers, in a 2007 exhibition named "The Writer's Brush: Paintings and Drawing by Writers".