The cyclist entered his plea by mail on Friday under a plea agreement with prosecutors, closing the case and avoiding a court appearance.
Armstrong's girlfriend, Anna Hansen, originally told police she was behind the wheel in the Dec. 28 accident in icy conditions but later confessed that she lied to avoid media attention.
Court records show Armstrong paid $238.50 to cover court fees and a $150 fine.
Deputy District Attorney Andrea Bryan says it is fairly common for people to reach plea deals in such cases and to enter pleas by mail. Careless driving can be punished by up to 90 days in jail but Bryan says jail time is rare.