Spitfire joined other light draft vessels of the squadron and boats from the heavier American warships in moving across the bar and up the Pánuco River and past the fort which guarded the stream.

On 13 December, Conner departed Tampico in Princeton and left Commander Tattnall in charge there until enough Army troops arrived to hold the town.

On 9 March, Spitfire led a flotilla of gunboats and other light draft naval vessels close to the shore to support the landing of Army troops who began to invest the city.

Just after dawn the following day, Spitfire anchored east of the fortress at San Juan de Ulúa and opened fire on the castle to divert the attention of the Mexicans from General Winfield Scott who shifted his headquarters ashore that morning.

At mid-afternoon on the 22nd, when the cannonading began, Spitfire led Josiah Tattnall III's flotilla in an attack on the shore end of the city walls and maintained the bombardment until dark.

That night, Spitfire's executive officer, Lt. David Dixon Porter, made a daring boat reconnaissance of the harbor at Vera Cruz to locate the best position for the flotilla when it resumed its shelling.