Gerald W. Johnson, 89, a journalist and historian who had been a colleague of H. L. Mencken and a speech writer for Adlai E. Stevenson, died of pneumonia Saturday in the Edgewood nursing home in Towson.

Mr. Johnson had been a contributing editor and columnist with the New Republic magazine since 1954. He also was an editorial writer for the Baltimore Sunpapers from 1926 to 1943.

He was the author of more than 30 books on the history and mores of the American people. These ranged from biographies of presidents Andrew Jackson and Franklin D. Roosevelt and a history of the Civil War to "A Little Night Music," which was a book on his hobby of flute playing.

His last two books were "The Imperial Republic," a 1972 work that was a somewhat pessimistic view of the foreign relations of this country, and "America-Watching." published in 1976. He was a critic of our involvement in the war in Vietnam.

Mr. Johnson was a speech writer for Democratic presidental candidate Stevenson during his unsuccessful races in 1952 and 1956 against Dwight D. Eisenhower.

Described as a southerner and remembered as a champion of Roosevelt's New Deal, Mr. Johnson preferred to described himself simply as a "realist." He wrote in 1954 that his view of the Republican Party was that "it is great when what is required is not thinking, but only bulldog tenacity." "

He went on to say that the role of the GOP in this country is similar to that of brakes on an automobile, but added, "the brakes have locked."

When a group of friends gave a surprise 70th birthday party for Mr. Johnson, Stevenson came and hailed Mr. Johnson as "the critic and conscience of our time. Every friend and reader of Gerald Johnson is in his debt for a thousand rescues from boredom in an age when humor is suspect and conformity a virtue."

Perhaps Mr. Johnson's least successful stint as critic and conscience came during a three-month period in 1938 when his friend H.L. Mencken was named acting editorial page editor of the Baltimore Evening Sun.

William Manchester in his book, "H.L. Mencken: Disturber of the Peace," wrote that the infamous period began with Mencken ordering Mr. Johnson to clear everything aside and to "read up on Prohibition" because he [Mencken] expected the "Dry Lobby" to make a comeback in Maryland.

A week later, saturated in the literature of prohibition, Mr. Johnson was ordered to abandon this field for the one of conservation. The admonition from Mencken was to study nothing else.

This went on for three months. Mr. Johnson became an authority in a variety of fields, but under Mencken's guidance never got one line in the paper.

A more satisfying collaboration beteween the two Baltimore journalists, along with fellow Sun writer Frank R. Kent, had been the 1937 book entitled "The Sunpapers of Baltimore."

He transferred from the Evening Sun to its sister morning publication, also as an editorial writer, in 1939. He left the Baltimore Sun to become a free-lance writer in 1943.

During a two-year span on Baltimore radio in the mid-1950s he won the DuPont Commentators', the Sidney Hillman Foundation and the George Foster Peabody awards.

His free-lance pieces included one in the American Scholar in 1970, which took the somewhat unorthodox view that, all else aside, at least vice presidents of the United States were interesting fellows.

He maintained that there was widespread opinion that the perfect model of a vice president was Alexander Throttlebottom in the Gershwin-Kaufman musical "Of Thee I Sing" who could not get a library card because two references were required.

But Mr. Johnson wrote that vice presidents had been colorful characters. Indeed, "One (Arron Burr) killed Alexander Hamilton and another (Richard M. Johnson) is reputed to have killed Tecumseh. One (John C. Breckinridge) commanded hostile trooops that came within sight of the church spires of Washington, and four broke with their own party and headed dissident movements."

Mr. Johnson was born in Riverton, N.C., and was a 1911 graduate of Wake Forest College. He worked for a number of North Carolina papers, including the Greensboro Daily News where he was the music critic, before serving with the 81st Infantry Division in France during World War I. He also taught journalism at the University of North Carolina for two years before coming to Baltimore in 1926.

He and his wife, Kathryn, made their home in Baltimore. Other survivors include two daughters, three sisters, and seven grandchildren.