From Wikipedia (accessed June 12, 2006):
Mad is known for the stability and longevity of its talent roster, billed as "The Usual Gang of Idiots," with several creators enjoying 30-, 40- and even 50-year careers in the magazine's pages.
According to the "Mad Magazine Contributor Appearances" website, nearly 800 contributors have received bylines in at least one issue of Mad, but only three dozen of those have contributed to 100 issues or more.
Al Jaffee has appeared in the most issues (471 as of June 2013). The other three contributors to have appeared in more than 400 issues of Mad are Sergio Aragonés, Dick DeBartolo, and Mort Drucker; Dave Berg, Paul Coker and Frank Jacobs have each topped the 300 mark. (The list calculates appearances by issue only, not by individual articles or overall page count; e.g. although Jacobs wrote three separate articles that appeared in issue #172, his total is reckoned to have increased by one.) Each of the following contributors has created over 150 articles for the magazine:
Dick DeBartolo Desmond Devlin Stan Hart Frank Jacobs Tom Koch Arnie Kogen Larry Siegel Lou Silverstone Mike Snider Writer-Artists: Sergio Aragones Dave Berg John Caldwell Duck Edwing Al Jaffee Peter Kuper Don Martin Paul Peter Porges Antonio Prohías
Artists: Tom Bunk Bob Clarke Paul Coker Jack Davis Mort Drucker Hermann Mejia Jack Rickard Angelo Torres Rick Tulka Sam Viviano Wally Wood George Woodbridge Tom Richmond
Some of the editorial staff, notably Charlie Kadau, John Ficarra, and Joe Raiola, have had dozens of bylined articles. They, along with Al Feldstein, Nick Meglin and others, also had creative input with many articles. Other notable contributors Among the irregular contributors with just a single Mad byline to their credit are Charles M. Schulz, Chevy Chase, "Weird Al" Yankovic, Andy Griffith, Will Eisner, Kevin Smith, J. Fred Muggs, Boris Vallejo, Sir John Tenniel, Jean Shepherd, Winona Ryder, Jimmy Kimmel, Jason Alexander, Walt Kelly, Rep. Barney Frank, Tom Wolfe, Steve Allen, Jim Lee, Jules Feiffer, Donald Knuth and Richard Nixon, who remains the only President credited with "writing" a Mad article. Contributing just twice are such luminaries as Tom Lehrer, Gustave Doré, Danny Kaye, Stan Freberg, Mort Walker and Leonardo da Vinci. (Leonardo's check is still waiting in the Mad offices for him to pick it up.) Frank Frazetta (3 bylines), Ernie Kovacs (11), Bob and Ray (12), and Sid Caesar (4) appeared slightly more frequently. In its earliest years, before amassing its own staff of regulars, the magazine frequently used outside "name" talent. Often, Mad would simply illustrate the celebrities' preexisting material. The magazine has occasionally run guest articles in which notables from show business or comic books have participated. In 1964, an article called "Comic Strips They'd Really Like To Do" featured one-shot proposals by cartoonists including Mell Lazarus and Charles M. Schulz. More than once, the magazine has enlisted popular comic book artists such as Frank Miller or Jim Lee to design and illustrate a series of "Rejected Superheroes." In 2008, the magazine got national coverage for its article "Why George W. Bush is in Favor of Global warming." Each of the piece's ten punchlines was illustrated by a different Pulitzer Prize–winning editorial cartoonist.