For TIME Magazine, MacNeil reported many cover stories, including on such members of Congress as Sam Rayburn, Mike Mansfield, Lyndon Johnson, the Kennedy brothers (John, Robert, and Ted), Everett Dirksen, Gerald Ford, Hubert Humphrey, William Fulbright, Robert Byrd, and Howard Baker.

He appeared on such programs as NBC’s “Meet The Press” and CBS’s “Face The Nation.” For three years starting in 1964, MacNeil made a weekly report, “MacNeil on Congress,” for the Eastern Educational Television Network, a program enlarged in 1967 into “Washington Week in Review” on National Public Television.

At the time of his death, MacNeil was completing a fourth book, tentatively titled Call The Roll: A Candid History of the United States Senate.

After his death in June 2008 Oxford University Press contacted Richard A. Baker, historian emeritus of the Senate and asked him to assist in completing this book.

In 1976 MacNeil served as chairman of the United States Assay Commission, a citizens’ group annually appointed by the President to test the validity of the government's coinage, a commission originally created by George Washington during his administration and abolished in 1977 by President Carter as an economy measure.