Following the end of the war, prominent Republicans in Whittier approached Nixon about running for Congress in 1946. Nixon accepted their offer, and, on November 6, 1946, defeated Democratic Congressman Jerry Voorhis by more than fifteen thousand votes. He moved to Washington with his wife Pat and their young daughter, Patricia (known as "Tricia"), who had been born on February 21, 1946. (Their second daughter, Julie, was born on July 5, 1948.)

As a congressman, he served on the Education and Labor Committee and supported the enactment of the Taft-Hartley Act, which greatly restricted the powers of labor unions. Nixon also served on the Herter Committee, which traveled to Europe to prepare a preliminary report on the Marshall Plan.

In 1948, as a member of the House Un-American Activities Committee (HUAC), he took the lead in investigating charges against former State Department official Alger Hiss of spying for the Soviet Union before and during World War II. The case turned the young congressman into a national figure—and a controversial one, because many prominent figures asserted Hiss's innocence. Not until decades later, after the end of the Cold War, would intelligence information released both by the U.S. government and the Russian government confirm Hiss' guilt.

Nixon was easily re-elected in 1948.