Beard, a great-grandson of Vice President John C. Breckinridge, was born into a well-to-do family and grew up at the Westbury Hotel on East 69th Street in Manhattan. His relationship with his parents was cool, to judge by his quip "I never saw my mother up close."

He attended the Taft School, where he was a leader at the humor magazine, and he decided to become a humorous writer after reading Catch-22.

He then went to Harvard University (from which he graduated in 1967) and joined its humor magazine, the Harvard Lampoon, which circulated nationally. Much of the credit for the Lampoon's success during the mid 1960s is given to Beard and Douglas Kenney, who was in the class a year after Beard's. In 1968, Beard and Kenney wrote the successful parody Bored of the Rings.

In 1969, Beard, Kenney and Rob Hoffman became the founding editors of the National Lampoon, which reached a monthly circulation of over 830,000 in 1974 (and the October issue of that year topped a million sales). One of Beard's short stories published there, "The Last Recall", was included in the 1973 Best Detective Stories of the Year. During the early 1970s, Beard was also in the Army Reserve, which he hated.

In 1975 the three founders cashed in on a buy-out agreement for National Lampoon; Beard got US$2.8 million and left the magazine. After an "unhappy" attempt at screenwriting, he turned to writing humorous books. Those that have reached the New York Times Best Seller list are Sailing: A Sailor's Dictionary (1981, with Roy McKie), Miss Piggy's Guide to Life (1981), Leslie Nielsen's Stupid Little Golf Book (1995, with Leslie Nielsen), French for Cats (1992, with John Boswell), and O.J.'s Legal Pad (1995, with John Boswell and Ron Barrett). Other notable books include Latin for All Occasions (1990), The Official Politically Correct Dictionary and Handbook (1992, with Christopher Cerf), and What's Worrying Gus? (1995, with John Boswell).

The New York Times has described Beard as "enigmatic". Among the enigmas, apparently, is his birthdate. Not even the year of his birth appears in the Library of Congress Cataloging-in-Publication data of his books or in various Web and print sources. However, Josh Karp's biography of Doug Kenney says that Beard was "a nearly thirty-year-old man" when he left the National Lampoon on March 18, 1975 and an article published on November 29, 1987, gives his age as 42. The birth year given above, 1945, is based on these two statements.

According to Josh Karp, Beard is remembered from his Harvard years as patrician, a pipe smoker, not over-concerned with the appearance or cleanliness of his clothes, misanthropic but not malicious, capable of understanding and organizing any subject, a gifted student who occasionally wrote parodic papers. He was prematurely mature and the Harvard Lampoon's arbiter. As a comic writer he excelled at parody, and his hero was S. J. Perelman. All these characteristics meant that he was an excellent partner with Kenney, who was flamboyant, fond of poses, and given to seeing humor where others recoiled.

Many of these characteristics, not just the clothes, continued into Beard's National Lampoon years. The comedy writer Chris Miller remembers that Beard "knew everything" and that he said on leaving the Lampoon that he was sick of being the father to all the writers. (Beard would have been about 30.) The comic writer and actor Tony Hendra says that at the beginning of Beard's tenure, he was painfully shy but the magazine's authority over what material was used. In the next few years, he went through "the greening of Beard", growing his hair, switching from cheap beer to expensive whiskey, and in 1974, forming a relationship with the writer Gwyneth Cravens.

In 1991, an article in a reliable publication said that Beard and Cravens divided their time between Manhattan and a renovated boat shed in East Hampton and referred to them as partners. A 2006 interview in a different publication said that Beard and Cravens had married. Also in 2006, Karp wrote that "reportedly" the couple had added California to their list of addresses and that Beard played golf almost daily but never kept score.