William French Smith, who served as Ronald Reagan's personal lawyer before joining Reagan's Cabinet as attorney general in 1981, died of cancer yesterday at University of Southern California Hospital in Los Angeles. He was 73.

A soft-voiced, white-haired New England native who maintained a relatively low profile in Washington, Mr. Smith was credited with spearheading the nation's attack on drugs and organized crime, and with helping ease the way for corporate mergers during his four years as head of the Justice Department.

Mr. Smith, a longtime partner in one of California's largest law firms, submitted his resignation to Reagan in January 1984. But difficulties surrounding the confirmation of his successor, Edwin Meese, kept him in office for a year longer.

Authorities said Mr. Smith was admitted Oct. 2 to Kenneth Norris Jr. Cancer Hospital at County-USC Medical Center. His family was with him when he died.

In a statement last night, Reagan praised Mr. Smith as an attorney general of talent, wisdom and the highest integrity, who served the nation with "the greatest dedication and distinction." He called him a valued and trusted friend and adviser, and said he "often sought his wise counsel."

As the nation's chief law enforcement officer and the government's principal legal adviser, Mr. Smith was considered an important participant in the Reagan administration's effort to impose a more conservative stamp on the nation's domestic policy.

But given Mr. Smith's quiet and undramatic manner, the extent to which he was involved in the changes of the Reagan years was not always readily apparent.

"It was one of the ironies of his tenure that it was a tenure characterized by such far-reaching and profound change in the direction of the federal legal system . . . {that} it was done in a quintessentially quiet, prudent and lawyerlike fashion," Solicitor General Kenneth Starr told the Associated Press.

Starr, who had been a partner in Smith's law firm and was his chief of staff at the Justice Department, called Mr. Smith "an immensely gifted lawyer with marvelous, sound and wise judgment."

As the day of his departure from the Justice Department approached in 1985, Mr. Smith said that one of his principal satisfactions was passage of the Comprehensive Crime Control Act of 1984. He called it the "most dramatic reform" of U.S. criminal law.

Mr. Smith acknowledged that civil rights policies instituted during his tenure had come under heavy fire and that the administration had "been accused of abandoning the federal civil rights effort."

Although there was a sharp policy turnabout in such controversial areas as busing and affirmative action, Mr. Smith said the enforcement of civil rights laws was vigorous under his tenure.

Indeed, he said, "during the last four years we have been more aggressive in enforcing the civil rights laws in almost every area than any administration."

But critics contended that there had been a decline in the number of cases filed in the areas of housing and educational discrimination.

Mr. Smith also asserted that his administration had inflicted severe damage on organized crime and had brought about a 100 percent increase in resources allocated to the war on drugs.

Credited with internationalizing the effort against crime, Mr. Smith traveled widely during his tenure, ranging as far afield as the Khyber Pass.

Under his administration, he said Justice shifted antitrust policy "back to the real economic world by focusing upon truly anticompetitive activities rather than outmoded and exotic theories."

Taking the position that "bigness does not necessarily mean badness," he was seen as having helped create the merger era of the 1980s.

As attorney general, he was influential in the nomination of many judges and was credited with helping choose Sandra Day O'Connor as the first woman on the U.S. Supreme Court.

His acceptance of a $50,000 severance fee from a California firm for which he performed legal work created controversy, and he later returned the money. A Justice Department investigation closed in 1982 without any finding that either law or departmental rules had been violated.

A descendant of one of the first presidents of Harvard University, Mr. Smith was born Aug. 26, 1917, in Wilton, N.H. His father was president of the Mexican Telephone and Telegraph Co., with headquarters in Boston.

He received a bachelor's degree from the University of California at Berkeley and a law degree from Harvard before serving as a naval officer during World War II. After the war he joined the Los Angeles law firm Gibson, Dunn & Crutcher, where he rose to senior partner.

He also became a member of the group who, impressed by Reagan's televised speech on behalf of Barry Goldwater in 1964, persuaded Reagan to run for governor of California two years later.

The group has been described as Reagan's Kitchen Cabinet, and Mr. Smith was called its chairman.

According to one news account, Reagan seldom made a significant move as governor without asking, "Has this been cleared with Bill Smith?"

Mr. Smith's first marriage ended in divorce. In 1964 he married his second wife, Jean Webb, with whom he lived in the Jefferson Hotel on 16th Street NW during his tenure in office. He had four children from his first marriage.