The Legion was formed in Breslau, Silesia in February 1807 from an infantry regiment and cavalry regiment in the service of the Kingdom of Naples that were descended from Jan Henryk Dąbrowski's Dąbrowski's Legions and Karol Kniaziewicz's Danube Legion originally raised in the 1790s. The new formation was expanded from the Neapolitan cadre into a formation of three infantry regiments and one cavalry regiment, initially named the Polish-Italian Legion as it had been organized around the Poles formerly in Italian service. Most of the recruits came from ex-Prussian and ex-Austrian territories, particularly Poznań and Pomerania.
The Polish-Italian Legion fought its first engagement at the siege of Klodzko and then was transferred to Kingdom of Westphalia in October 1807 and was placed in garrison in the capital, Kassel where it was recruited to full authorized strength from Poles in French occupied territory. The newly expanded cavalry regiment arrived in Kassel on November 11, 1807.
Napoleon I directed that the Legion be transferred to French service on February 21, 1808. The Legion was transferred to Poitiers, France and was officially renamed the Vistula Legion on March 31, 1808 with the equivalent status of French line units. The infantry was reorganized to the 1808 pattern of six company battalions in April of that year in conformance with the new French organization decreed on February 18, 1808. The Legion's depot was at Sedan. All of the personnel of the Legion were to be of Polish ethnicity except for the company clerks, the fourriers, battalion adjutant non-commissioned officers, and paymasters. who were to be French. The strength of the Legion was set at 5,959 men in June 1808. The 2nd and 3rd infantry regiments of the Vistula Legion in June 1808 and participated in Napoleon's invasion of Spain (the Peninsular War). On March 24, 1809 at the Battle of Yevenes, 600 of Polish lancers of the Legion of the Vistula lost all their banners (which had not been defended because nobody knew they were hidden in the carriage) when they had cut through the ambush set by 5,000 of Spaniards. The regiment was renamed in 1811 into the 7e régiment de chevau-légers lanciers.
Napoleon I ordered a Second Vistula Legion formed from prisoners taken after his defeat of the Austrians at the Battle of Wagram in July 1809. Recruiting was slow and only two battalions were raised and were sent to Sedan in October 1809. Unlike the original legion, ethnic Germans were accepted into the new formation. The Second Vistula Legion was unable to recruit up to strength and was disbanded in February 1810 with its personnel being amalgamated with the original Legion as its 4th Regiment.
On February 7, 1811, a second lancer regiment was raised, and on June 18 of that year, the two lancer regiments were removed from the legion and redesignated as the 7th and 8th Chevauleger-lancier regiments of the French line, with six converted French dragoon regiments being numbered 1 through 6.