Deborah Ragland Marshall, 31, the only black woman and the youngest person ever elected to the Prince George's County Council, announced yesterday that she will not seek reelection this fall. Marshall said "the only reason to run again would be as a springboard to a political career, and that's something I don't want."

Marshall, who was elected to an at-large seat in 1978, said that she plans to move to Philadelphia at the end of her term in December and look for a job in private industry. She also said she expects to marry Philadelphia council member John White, 33, a rising political star in that city. She added that her decision not to run was made independently of her personal plans.

"I did not intend to go into politics, and right now I'm looking for something where I can mix all the things that I'm interested in, politics, public relations, marketing," said Marshall, who was raised in Washington and holds a master's degree in counseling from Howard University. She served as Prince George's cochairman of the successful 1980 presidential primary effort of Sen. Edward M. Kennedy.

A self-proclaimed "slate-baby," whose political career was spawned and nurtured by the county's once dominant Democratic organization, Marshall became increasingly outspoken and independent over the last year. For example, while she began her career in Prince George's in 1974 as an aide to former County Executive Winfield M. Kelly, Jr., she consistently voted against a Kelly-led cable firm in its drive to obtain a lucrative cable television franchise last fall. During the months of deliberation, Marshall refused to take Kelly's phone calls, saying she was complying with the county's prohibition on private contacts with the cable firms. Her behavior so angered her former employer that by the end of the year, Kelly made clear to his associates that he would not support Marshall if she ran for reelection.

Marshall was the third highest vote-getter in a field of 15 in the 1978 Democratic primary and, in the general election, ran fifth among six successful candidates, to become the second black elected to the council, which she currently serves as vice chairman. As a result of councilmanic redistricting, Marshall, who lives in Suitland, and council member Sue V. Mills were placed in the same district this year, and with a population that is 80 percent black, Marshall was expected to win reelection easily.

Mills announced two weeks ago that she plans to move into another district, into which many of her Oxon Hill constituents were switched. The only announced candidate for Marshall's seat is Robert A. Spencer of Seat Pleasant, who ran unsuccessfully against state Sen. Tommie Broadwater in 1974.