IN THE DISTRICT of Columbia's painfully slow transformation from an Old South colony run by Congress to an urban area with its own elected government, perhaps no man had more influence than John Lanneau McMillan of Florence, S.C. As chairman of the House District Committee for two decades, Mr. McMillan, who died in his home town on Monday, held-- and used to the hilt-- ultimate authority over almost every aspect of life in the city. If the city only now is discovering the ways that a responsible local government can flex its muscles to effect constructive change, it is because of Mr. McMillan's relentless, paternalistic preoccupation with tis affairs.

Comfortably flanked for years by other rural southern lawmakers who shared his views about black people and his inability to appreciate the needs of a large urban area, Chairman McMillan was exceptionally successful in fighting off an outbreak of local democracy. Not until his own constituents finally retired "Johnny Mac" from Congress in 1972-- largely because of his preoccupation with District affairs and because a majority of the black voters supported an opponent -- could the House consider a D.C. home-rule bill willingly cleared by the District Committee; it was enacted later in 1973.

Still, in Congress, in the South and in this city, changes had taken place that contributed to the District's transition from a ward of Uncle Sam to a city with its own elected mayor and city council. Even though Mr. McMillan thwarted an all-out effort by President Johnson to enact a D.C. home-rule bill, the movement to self-government only slowed; two years later, the president-- together with a Congress that cared less and less about the municipal affairs of the District-- allowed an appointed "mayor" and "city council."

Whatever criticisms would evolve about the cautious manner of Walter Washington as an elected official after 1975, it was Mr. Washington's circumspect, dedicated stewardship that worked to convince Congress that the people in this city could and should exercise more control over their local affairs.That exercise, in turn, produced a desire in the community for yet another change in the government-- to a more vigorous city administration under a new mayor.

Shortly before his defeat, John McMillan was still arguing that the "interloper" here was not the federal establishment, "but rather the thousands of persons who have swarmed here, most of whom add little or nothing to the day-by-day operations of the federal government, who are 'local' in character and needs . . . " Throughout those years, of course, the opinions of Washingtonians had no standing; it took a decision by the people in his home district to officially mark the end of an era in District of Columbia history.