Despite a long history of criticizing Marxism, MacLeish came under fire from conservative politicians of the 1940s and 1950s, including J. Edgar Hoover and Joseph McCarthy. Much of this was due to his involvement with anti-fascist organizations like the League of American Writers, and to his friendships with prominent left-wing writers. In 1949 MacLeish became the Boylston Professor of Rhetoric and Oratory at Harvard. He held this position until his retirement in 1962. In 1959 his play J.B. won the Pulitzer Prize for Drama. From 1963 to 1967 he was the John Woodruff Simpson Lecturer at Amherst College. Around 1969/70 he met Bob Dylan, who describes this encounter in the third chapter of Chronicles, Vol. 1.

MacLeish greatly admired T. S. Eliot and Ezra Pound, and his work shows quite a bit of their influence. In fact, some critics charge that his poetry is derivative and adds little of MacLeish's own voice[citation needed]. MacLeish's early work was very traditionally modernist and accepted the contemporary modernist position holding that a poet was isolated from society. His most well-known poem, "Ars Poetica," contains a classic statement of the modernist aesthetic: "A poem should not mean / But be." He later broke with modernism's pure aesthetic. MacLeish himself was greatly involved in public life and came to believe that this was not only an appropriate but an inevitable role for a poet.

MacLeish worked to promote the arts, culture, and libraries. Among other impacts, MacLeish was the first Librarian of Congress, to begin the process of naming, what would become the United States Poet Laureate. The Poet Laureate Consultant in Poetry to the Library of Congress, came from a donation in 1937 from Archer M. Huntington, a wealthy ship builder. Like many donations it came with strings attached. In this case Huntington wanted the poet, Joseph Auslander to be named to the position. MacLeish found little value in Auslander’s writing. While, MacLeish was happy that having Auslander in the post attracted many other poets, such as, Robinson Jeffers and Robert Frost, to hold readings at the library. He set about establishing the consultantship as a revolving post rather than a lifetime position. In 1943, MacLeish displayed his love of poetry and the Library of Congress by naming Louise Bogan to the position. Bogan who had long been a hostile critic of MacLeish’s own writing, asked MacLeish why he appointed her to the position. MacLeish replied that she was the best person for the job. For MacLeish promoting the Library of Congress and the arts was vitally more important than petty personal conflicts.

It was in a June 5, 1972 issue of The American Scholar that MacLeish laid out in an essay his philosophy on libraries and librarianship, further shaping modern thought on the subject. MacLeish remarked in the essay that libraries are more than a mere collection of books. "If books are reports on the mysteries of the world and our existence in it, libraries remain reporting on the human mind, that particular mystery, still remains as countries lose their grandeur and universities are not certain what they are." For MacLeish, libraries are a massive report on the mysteries of human kind.

Two collections of MacLeish's papers are held at the Beinecke Rare Book and Manuscript Library: these are the Archibald MacLeish Collection (YCAL MSS 38) and Archibald MacLeish Collection Addition (YCAL MSS 269).

Joseph Kramm (1952) · William Inge (1953) · John Patrick (1954) · Tennessee Williams (1955) · Albert Hackett / Frances Goodrich (1956) · Eugene O'Neill (1957) · Ketti Frings (1958) · Archibald MacLeish (1959) · Jerome Weidman / George Abbott / Jerry Bock / Sheldon Harnick (1960) · Tad Mosel (1961) · Frank Loesser / Abe Burrows (1962) · Frank D. Gilroy (1965) · Edward Albee (1967) · Howard Sackler (1969) · Charles Gordone (1970) · Paul Zindel (1971) · Jason Miller (1973) · Edward Albee (1975)

Complete list: (1918-1925) · (1926-1950) · (1951-1975) · (1976-2000) · (2001-present)

John J. Beckley (1802) · Patrick Magruder (1807) · George Watterston (1815) · John Silva Meehan (1829) · John Gould Stephenson (1861) · Ainsworth Rand Spofford (1864) · John Russell Young (1897) · Herbert Putnam (1899) · Archibald MacLeish (1939) · Luther H. Evans (1945) · Lawrence Quincy Mumford (1954) · Daniel J. Boorstin (1975) · James H. Billington (1987)