John "Jock" Elliott Jr., 84, former chairman of the advertising agency Ogilvy & Mather and an authority on the history of Christmas, died of a cerebral hemorrhage Oct. 29 at a hospital in Mount Kisco, N.Y.

After heading the agency's American operations for a decade, Mr. Elliott chaired its worldwide division from 1975 until he retired in 1982.

Under his leadership, the agency won accounts including IBM and American Express, and more than tripled its earnings, to $2 billion.

"Big ideas are so hard to recognize, so fragile, so easy to kill," he said in 1981 when he announced his retirement. "Don't forget that, all of you who don't have them."

After Mr. Elliott became an advertising executive, he decided to focus his penchant for collecting on Christmas-related ephemera.

His trove included the manuscript of "The Christmas Carol" that author Charles Dickens used at public readings and the 17th-century book in which the words "merry" and "Christmas" are said to have first appeared together.

The collection of more than 3,000 pieces formed the basis of Mr. Elliott's book, "Inventing Christmas: How Our Holiday Came to Be," published in 2002.

Born and raised in New York City, Mr. Elliott was named for his investment counselor father. His mother, Audrey, sold real estate.

After graduation from Harvard University in 1942, he served in the Marine Corps during World War II. After returning home, he entered the ad business as a $60-a-week copywriter.

In 1986, he gave his wife the small island of Staffa off the coast of Scotland for her 60th birthday and arranged for her to be the "Laird of Staffa" for a few days before donating the island to the National Trust for Scotland.

Survivors include his wife of 49 years, Eleanor Thomas Elliott of Cross River, N.Y.; and a brother, Osborn Elliott, a former editor of Newsweek magazine, of New York.