Disclosed herein are systems and methods for carrier aggregation and fast network-switching with a single-baseband modem, carrier-aggregation-capable wireless-communication device (WCD). One embodiment takes the form of a system that includes a first RF integrated circuit (RFIC), a second RFIC, a baseband processor coupled to the first and second RFICs, a first carrier-aggregation circuit, a fast-network-switching connection manager coupled to the baseband processor, and a mode controller coupled to the baseband processor and to the fast-network-switching connection manager. The mode controller is configured to selectively place the wireless-communication device in a carrier-aggregation mode or in a fast-network-switching mode. In the carrier-aggregation mode, the WCD is operable to conduct carrier aggregation using at least the first carrier-aggregation circuit. In the fast-network-switching mode, the WCD is operable to conduct fast network switching using the fast-network-switching manager.