Galvez let an American force through New Orleans before Spain joined the cause. Gálvez was sent to Florida by New Spain Viceroy Martín de Mayorga, at the head of an expedition of colonial troops to aid American colonists in their rebellion against Britain. Spain’s motive was the chance to recover territories lost to the British, particularly Florida, and to remove the on-going British threat.

On June 21, 1779 Spain declared war on England. On June 25, 1779 a letter from London marked secret and confidential, went to General John Campbell of Strachur at Pensacola from King George III and Lord George Germain. General John Campbell was instructed that it was the object of greatest importance to organize an attack upon New Orleans. If General John Campbell thought it was possible to reduce the Spanish fort at New Orleans, he was ordered to proceed immediately to make preparations. These preparations included: (1) secure from Vice-Admiral Sir Peter Parker as many armed vessels as could be spared from Jamaica, (2) collect all forces which could be drawn together in the province, (3) take as many faithful Indians as the Superintendent could supply, (4) draw on the Lords Commissioners of the Treasury for all expenses.

 
Spanish troops storm the British positions at Pensacola.As an unfortunate twist of fate for General John Campbell, upon which his whole career was decided, this secret communication fell into the hands of Governor Galvez. After reading the communication from King George III and Germain, Gálvez, Governor of Louisiana swiftly and secretly organized Louisiana and New Orleans for war.

Gálvez carried out a masterful military campaign and defeated the British colonial forces at Manchac, Baton Rouge, and Natchez in 1779. The Battle of Baton Rouge on September 21, 1779 freed the lower Mississippi Valley of British forces and relieved the threat to the capital of Louisiana, New Orleans. In 1780, he recaptured Mobile from the British at the Battle of Fort Charlotte.

His most important military victory over the British forces occurred May 9, 1781, when he attacked and took by land and by sea Pensacola, the British (and formerly, Spanish) capital of West Florida from General John Campbell of Strachur. The loss of Mobile and Pensacola left the British with no bases in the Gulf of Mexico, except for Jamaica. In 1782, he captured the British naval base at New Providence in the Bahamas.

He received many honors from Spain for his military victories against the British, including promotion to lieutenant general and field marshal, governor and captain general of Louisiana and Florida (now separated from Cuba), the command of the Spanish expeditionary army in America, and the titles of viscount of Gálveztown and count of Gálvez.

The American Revolution ended while Gálvez was preparing a new campaign to take Jamaica.

The importance of Galvez’s campaign from the American perspective was that he denied the British the opportunity of encircling the American rebels from the south, and kept open a vital conduit for supplies. Galvez also assisted the American revolutionaries with supplies and soldiers, a good deal of it through intermediary and committed revolutionary, Oliver Pollock.

Gálvez, who saw it convenient for France and Spain to advance the cause of the American revolutionaries, was among those who drafted the terms of the Treaty of Paris (1783) that ended the war. By the treaty Spain officially regained East and West Florida from the British.

In recognition of his work and help to the American cause, George Washington took him to his right in the parade of July 4th and the American Congress cited Gálvez for his aid during the Revolution.

Galveston, Texas and St. Bernard Parish, Louisiana were both named after him. The Louisiana parishes of East Feliciana and West Feliciana were named after his wife Marie Felice de Saint-Maxent Estrehan.

The Cabildo, a branch of the Louisiana State Museum located on Jackson Square, New Orleans, Louisiana in New Orleans, has a portrait of General Galvez accompanied by a display of biographical information.

In 1911 in Galveston, TX, a hotel was built and named after him.