House Speaker John Boehner administered Hahn's oath.
Hahn will represent California's 36th Congressional District. She defeated Republican opponent Craig Huey earlier this month in a special election to replace Jane Harman.
Hahn had served in the City Council since 2001. Her father was an L.A. County supervisor and her brother is a former mayor of L.A.
Hahn's departure marks the first time in 64 years that a member of the Hahn family would not be in elected office in L.A.
City News Service contributed to this story.