From his age two onward, Jacob was raised in the Vääksy (Swedish: Växiö) manor, Kangasala, Finland (then part of Sweden proper) by his grandmother Karin Hansdotter, the mistress of King John III.

In 1605, in the prelude to the Polish–Russian War, the Polish–Lithuanian Commonwealth unofficially invaded Russia in support of False Dmitry I against the unpopular crowned tsar Boris Godunov, seeking to exploit the country's weakness for their own gain.

Godunov died in June 1605 and was replaced by False Dmitry I, whose popularity among the Russian populace declined rapidly during his reign, and the Polish withdrew when he was eventually murdered during an uprising in Moscow in May 1606.

Despite this, Russia's instability continued to the near-total breakdown of order, prompting the Polish to invade again in 1607 in support of the new usurper, False Dmitry II.

In 1609, the Tsar of Russia at the time, Vasili IV, approached King Charles IX of Sweden to form a military alliance against False Dmitry II and the Polish occupiers.

The two signed the Treaty of Viborg (Finnish: Viipuri), in which Russia ceded Kexholm County and the strategic Korela Fortress to Sweden in exchange for military support.

Thus, in accordance with the Swedish-Russian military alliance formed in 1609, he together with Evert Horn now took charge of providing an auxiliary corps to support the Russian forces commanded by Mikhail Skopin-Shuisky.

A combined Russo-Swedish army of about 10,000 soldiers set out from Novgorod in April 1609 and marched towards Moscow, defeating rebel forces and relieving the Siege of Troitse-Sergiyeva Lavra on their way.

[6] De la Gardie also claimed that Sweden should take advantage of the ongoing turmoil in Russia, later known as the Times of Trouble, and try to place Charles Philip, younger brother of the Swedish King Gustavus Adolphus, on the Russian throne.

Although the Marshal's Office came under criticism that year, De la Gardie continued to operate effectively, making large profits from leasing royal revenues and from loans to the crown.

During the Ingrian War, the Finnish soldiers nicknamed their commander Laiska-Jaakko ("Lazy Jacob"), due to the unusually lengthy six-year occupation of Novgorod.