Republican Charles W. Long '62, surprising everyone but his own campaign staff, swept to victory yesterday in a contest for the seat of Boston's traditionally Democratic Ward 21 in the Massachusetts state House of Representatives.

The 23-year-old second-year student in the Law School defeated 40-year-old Peter J. Cloherty by an unofficial margin of about 2,100 to 900, according to Long headquarters. He will be sworn into the legislature in a few days and will serve until January, 1965. The special election was held to fill the unexpired term of Democrat William F. Joyce, who was killed in an accident in October.

Long said last night he will remain a student in the Law School while he serves in the legislature, admitting that "it will be very hard" to keep up with both. Dazed by his victory and the newspaper and television reporters who beseiged him at his crowded headquarters last night, he withheld any policy statements until today.

Won Primary Easily

The district Long will represent at the State House includes parts of Allston, Brighton, and the Back Bay section of Boston. Long moved to an Allston apartment in September, 1962, and had barely fulfilled the one-year residence requirement for candidates for the legislature when he filed his candidacy. He won the Dec. 17 Republican primary with ease.

Campaign manager for Long was Howard Phillips '62, last president of the now-defunet Harvard Student Council. He described the campaign effort last night as one in which everyone in the district had been reached "in person, by mail, and by phone."

Long, a former member of Young Americans for Freedom, has described himself as a "moderate conservative" and a "Nixon Republican." He centered his campaign around opposition to a legislative pay raise and vigorous support for the Massachusetts Crime Commission. He was endorsed by state Attorney General Edward W. Brooke and U.S. Senator Leverett Saltonstall '14. Long, who lived in Adams House as a student at the College, is married and has a four-month-old son.

Cloherty, who served two terms in the House and was endorsed by House Speaker John W. McCormack, had no statement last night. He is a former director of the Massachusetts World's Fair Commission.