He was with The New York Times from 1977 until July, 2003, and served as its national political correspondent for the 1980 presidential election, polling editor from 1983 to 1990, political editor for George H. W. Bush's presidential campaign in 1988, and chief Washington correspondent from 1999 through 2003.

Clymer covered the 2000 presidential campaign for the Times and wrote several articles that were considered unfavorable by the Bush campaign.

Clymer may be best known for an incident on September 4, 2000, when Bush and running mate Dick Cheney appeared at a campaign event at Naperville, Illinois. While on stage before the event, Bush said to Cheney, "There's Adam Clymer, major league asshole from the New York Times." Cheney responded, "Oh yeah, he is, big time." The remarks were picked up by a live microphone, causing a minor campaign controversy. Bush later publicly stated "I regret that a private comment I made to the vice presidential candidate made it through the public airways. I regret everybody heard what I said."

While he never apologized for the comment itself, Bush made an attempt to smooth it over, making light of it at the next Washington Press Club Foundation Dinner by referring to Adam Clymer as a "major league ass...et." For his part, Clymer noted that Bush sent him a nice letter of condolences when his mother died in 2001.

In 2004, Clymer became a visiting scholar at the Annenberg Public Policy Center at the University of Pennsylvania, where he served as Political Director for the National Annenberg Election Survey.

In 1981, Clymer co-authored Reagan: The Man, the President with fellow New York Times journalists Hedrick Smith, Leonard Silk, Robert Lindsey, and Richard Burt. In 1999, he wrote Edward M. Kennedy: A Biography.

Born to children's book author Eleanor Clymer (née Lowenton) and Kinsey Clymer, Clymer attended Harvard College, receiving an A.B. in 1958. Clymer's journalism career began when he was in high school; he wrote for the school newspaper and collected sports scores for The New York Times. He did post-graduate work at the University of Cape Town, South Africa. In 1960, he joined The Virginian-Pilot in Norfolk, a job which he followed up with work at The Baltimore Sun and the New York Daily News.

Clymer has been married since 1961 to his wife, Ann. They had one daughter, Jane Emily Clymer, who was killed at the age of 18 by a drunken driver in September, 1985. The Clymers established a memorial scholarship at the University of Vermont in her name.