Karl Marlantes was born in Astoria, Oregon on December 24th, 1944. His father Leo served in the military and taught high school; his mother Elna was a waitress and bookkeeper. Karl attended Seaside High School, where he served as student body president, and graduated in 1963. Karl was awarded a National Merit Scholarship and attended Yale University. There, he was president of the Yale Rugby Club and played on the school rugby and football teams. He graduated Cum Laude in 1967 with a BA in Economics and was awarded a Rhodes Scholarship. After Yale, he attended Oxford for one semester before enlisting in the US Marine Corps, serving as a First Lieutenant in Vietnam, where he earned a Navy Cross, the Bronze Star, two Naval Commendation Medals for valor, and two Purple Hearts. He was discharged in 1972, completed a master's degree, and returned to the United States, where he married Edith Gisele Fitch, had five children, and became a corporate consultant. In part due to his untreated PTSD, Karl and Gisele divorced after 30 years of marriage. Marlantes started counseling at a Veteran's Affairs hospital and was officially diagnosed with the condition. In 2007, he married his current spouse, Anne Boswell.

Marlantes has published four books to date: (2009), (2011), (2019), and (2024). His first novel, , was published in a small edition of 1,000 copies in 2009 by El León Literary Arts of Berkeley, California. With an official publication date of May 1, advanced readers' copies were mailed out early in the year. Staffers at Barnes & Noble were impressed with the novel and submitted it to a first-novel contest, which led a number of New York publishers to express interest in obtaining the rights to the book. Eventually, Grove Atlantic Press made a deal with El León and copies already printed but not yet distributed were issued as galleys for a El León/Grove joint revised publication, which was issued in 2010. Matterhorn was Amazon's Book of the Month for March 2010 and debuted on the New York Times bestseller list in April 2010. It won the 2011 William E. Colby Award, was ranked #7 for Fiction in Time Magazine's Best Books of the Year (2010), and was one of the New York Times Notable Books of the Year (2010). It was also an ALA Notable Book (2010), won the Pacific Northwest Booksellers Association Award (2011), and won the Indies Choice Book Award (Adult Debut, 2011).

Marlantes' memoir, , published in 2011, which details Marlantes' experiences in Vietnam and reflects on the spiritual and psychological impacts of warfare, has sold over 250,000 copies. and depart from Marlantes' previous works' Vietnam War themes. is a historical epic which tells the story of Finnish immigrants in the Pacific Northwestern United States and their role in the US's labor movement. It is inspired by Marlantes' upbringing on the west coast and his family's Finnish ancestry. follows Louise Koski, the Finnish wife of World War II soldier Arnie, and takes place at the beginning of the Cold War.