Feinstein was elected to the U.S. Senate in a special election, alongside Barbara Boxer, marking a historic moment for California as the first time the state had two female senators.

Feinstein's service on the Senate Judiciary Committee was instrumental in shaping legislation related to crime, justice, and judicial appointments. She played a role in numerous confirmation hearings for Supreme Court justices.

Feinstein was a leading advocate for gun control and authored the Federal Assault Weapons Ban, which was enacted in 1994. This law prohibited the manufacture, transfer, and possession of certain semi-automatic assault weapons and large-capacity magazines.

Feinstein served as Chairwoman of the Senate Intelligence Committee, overseeing the nation's intelligence agencies and playing a critical role in national security oversight. This position gave her access to classified information and significant influence over intelligence policy.

Under Feinstein's leadership, the Senate Intelligence Committee released a heavily redacted report detailing the CIA's use of enhanced interrogation techniques (torture) following the 9/11 attacks. The report sparked intense debate and controversy.

Feinstein became the ranking member of the Senate Judiciary Committee, placing her at the forefront of judicial confirmations, including that of Justice Brett Kavanaugh.

At the time of her death, Dianne Feinstein was the longest-serving female U.S. Senator, having represented California for over three decades.