Lowell's buoyant years saw the issue of For the Union Dead (1964), which showcases one of the most anthologized titles, and the Obie-winning play The Old Glory (1965), a trilogy based on Nathaniel Hawthorne's "My Kinsman, Major Molineux" and Herman Melville's novella Benito Cereno. During this vigorous, assertive era of the Vietnam War, Lowell produced Near the Ocean (1967), two dramas; Prometheus Bound (1967); Endecott and the Red Cross (1968); and Notebook 1967–1968 (1968), a diary in unrhymed sonnet form that lauds colleagues Allen Tate, John Crowe Ransom, Randall Jarrell, and T. S. Eliot. Following Lowell's marriage to his third wife, British author Lady Caroline Blackwood, and the birth of a son, he found hope in lithium treatment. He began detailing the emotional crisis and renewal in a deeply allusive sonnet series entitled The Dolphin (1973), winner of a second Pulitzer Prize.
To his detriment, Lowell explored personal events in indiscreet verse, which he performed at public readings. A final collection, Day by Day (1977), a pensive series weakened by obscurity and repetition, won a National Book Critics Circle award. Imitations, containing modernizations of Homer, Sappho, Rilke, Villon, Mallarme, and Baudelaire, won the 1962 Bollingen Prize; Poetry (1963) received a Helen Haire Levinson Prize. Shortly after abandoning England and his wife to return to Elizabeth Hardwick, on September 12, 1977, Lowell died unexpectedly of congestive heart failure in a New York City taxi. He was eulogized at Boston's Episcopal Church of the Advent and buried among his ancestors. Collected Poems was issued in 1997.
