Alaska Airlines Mileage Plan miles do not expire (kind of-keep reading). However, Mileage Plan accounts that are inactive for two years or more are deactivated and must be reactivated in order to access Mileage Plan Miles in such accounts. Activity in an account includes redeeming a Mileage Plan award or accruing mileage in the Alaska Airlines Mileage Plan program.
A deactivated Mileage Plan account can be reactivated, and any miles in the deactivated account can be reinstated, for up to a year after deactivation. If your Mileage Plan account is locked due to inactivity, simply call customer care to verify your identity, and they’ll reactivate the account for you at no charge. If a deactivated Mileage Plan account is not reactivated within 1 year after deactivation (3 years after your last qualifying activity), it may not be reactivated in the future, and all Mileage Plan miles previously associated with that account will be forfeited.