Malmborg was born at the Rågåkra farm in Kräklingbo parish on the Baltic Sea island of Gotland in Sweden.
He was the son of Captain Pehr Gustaf Malmborg (1777–1828), who had been decorated with the gold medal for bravery after the Battle of Svensksund in 1790, before he was teenager.
[2] President Abraham Lincoln formally recognized Malmborg as the vice consul of Norway and Sweden at Chicago on November 22, 1861.
In an official report on April 10, 1862, Colonel Stuart wrote of Malmborg, "He instantly perceived the aim of every movement made by the enemy; he could advise me quickly and prudently how to use my men.
On January 1, 1865, Malmborg was commissioned a colonel in the First Veteran Army Corps under General Winfield Scott Hancock and was ordered to oversee the recruiting in Illinois, with headquarters in Chicago once again.