John J. Sexton, 52, a Washington tax lawyer since 1958 who was active in civic organizations and Democratic Party affairs in Montgomery County, died of cancer Dec. 27 at his home in Chevy Chase.

Mr. Sexton was chairman of the Montgomery County Democratic Central Committee from 1972 to 1974. He was county campaign manager for the successful races of Paul S. Sarbanes for the Senate in 1976 and Charles Gilchrist for county executive in 1978.

An alternate delegate to the 1980 Democratic Party National Convention, he managed president Carter's reelection campaign in Montgomery County.

He was vice chairman of the Montgomery County Charter Review Commission from 1979 to 1982 and a past president of the Tulip Hill Citizens Association.

Mr. Sexton joined the Washington law firm of Arent, Fox, Kintner, Plotkin & Kahn in 1958. He became a partner in 1962.

He was a former chairman of the Federal Bar Association's taxation council, a member of the American Bar Association's taxation section, a fellow ot the American College of Tax Counsel, a contributing editor to the Journal of Real Estate Taxation, and a consultant to the American Law Institute.

A native of Arlington, Mass., he earned bachelor's and law degrees at Harvard University. He moved to this area in 1953. He was a clerk with the U.S. Court of Appeals and a trial attorney with the Justice Department before joining Arent, Fox.

Survivors include his wife, Janet H., a son, Adam D., and a daughter, Diana H., all of Chevy Chase, and a sister, Mary Sexton of Arlington, Mass.