Born August 16, 1926 in Phoenix, Arizona, to Lewis Barrett Welch Sr. and Dorothy Brownfield Welch. Mrs. Welch was the daughter of a wealthy Phoenix surgeon. Following the birth of Welch's sister his parents marriage broke up, and Dorothy Welch moved the family to California in 1929. Welch was three at the time, and for most of his childhood his mother moved from town to town in California. He attended schools in Santa Monica, Coronado, La Mesa, and El Cajon. In Palo, Alto Welch finished high school.
Welch entered Reed College in 1948, and the following year moved into a house with Gary Snyder; the following year they were joined by Philip Whalen. By the fall of 1949 Welch was co-editor of the school's literary magazine and was writing constantly. He wrote his senior thesis on Gertrude Stein and graduated in 1950.
For a number of years Welch showed his poetry only to close friends. With the emergence of the Beat movement, however, Welch's friends Philip Whalen and Gary Snyder began receiving national attention. Welch's desire to devote himself completely to his poetry was revived. He transferred to the Oakland office of Montgomery Ward and soon became a part of the San Francisco poetry scene. In 1958 he was fired from his job. His marriage fell apart soon after.
At the same time, however, Welch's poetry was beginning to meet with some success. Donald Allen included one of Welch's poems in The New American Poetry - the important anthology published in 1960. That same year Welch's first book, Wobbly Rock, was published. He was drinking heavily during this time, but he continued to write extensively. For a time he lived with his mother in Reno, Nevada, and then in a cabin in the Trinity Alps. He moved back to San Francisco in 1963, and in 1965 published three books.
In 1965, Welch began teaching a poetry workshop offered through the Extension program of the University of California at Berkeley. Despite his burgeoning success, Welch's bouts with depression and heavy drinking continued. After the breakup of another relationship in 1971 Welch returned to the mountains. On May 23, 1971, Gary Snyder went up to Welch's campsite and found a suicide note in Welch's truck. Despite an extensive search, Welch's body was never recovered.