Agricultural expansion and the establishment of Willmar as a division point on the Great Northern Railway determined the growth of the community. The first settlers arrived during the 1850s attracted to the fertile land and an abundance of timber and game. The Dakota War of 1862 left the township abandoned for several years. The advent of the railroad in Kandiyohi County in 1869 brought new settlers. Many were of Swedish and Norwegian origins; hence the predominance today of residents of Scandinavian heritage. In 1870, Leon (Chadwick) Willmar, a Belgian acting as an agent for the European bondholder of the St. Paul and Pacific Railroad, purchased the title to Section 1 of Willmar Township. Willmar was established as the county seat in 1871 and was incorporated as a village in 1874 and as a city in 1901.
Willmar was the site of The Great Willmar, Minnesota Raid, a famous bank robbery by the Machine Gun Kelly gang on July 15, 1930. They robbed the Bank of Willmar (now Bremer Bank) of about $70,000, and wounded 3 people during the robbery.
From 1977 to 1979, Willmar was the site of the Willmar 8, a famous strike of female workers confronting sexual discrimination at a local bank. The story of the strike was featured in mainstream media and made into a documentary.
Willmar native and composer, Bradley Joseph, often references the past when he names his songs and his music is frequently reminiscent of his rural Minnesota roots. "Wind Farmer" was inspired by childhood visits to a relative's farm near Olivia, and his company, Robbins Island Music, is named after a city park in Willmar, says Anne Polta of the West Central Tribune.
Willmar is home to the annual Sonshine Festival, a Christian music festival.