Karl Ludwig Leimbach was a German educator and literary historian.

Karl Ludwig Michelet was a German philosopher. He was born and died in Berlin.

Karl Ludwig Freiherr von Pöllnitz was a German adventurer and writer from Issum.

Karl Mannheim was an influential Hungarian sociologist during the first half of the 20th century. He is a key figure in classical sociology, as well as one of the founders of the sociology of knowledge. Mannheim is best known for his book Ideology and Utopia (1929/1936), in which he distinguishes between partial and total ideologies, the latter representing comprehensive worldviews distinctive to particular social groups, and also between ideologies that provide outdated support for existing social arrangements, and utopias, which look to the future and threaten to transform a society.

Károly Mária Kertbeny was a Hungarian journalist, translator, memoirist, and human rights campaigner. Kertbeny coined the words heterosexual and homosexual as the German nouns Heterosexual and Homosexual.

Karl Arthur Marlantes is an American author and Vietnam War veteran. He has written three books: Matterhorn: A Novel of the Vietnam War (2010), What it is Like to go to War (2011), and Deep River (2019).

Karl Heinrich Marx was a German-born philosopher, economist, historian, sociologist, political theorist, journalist, critic of political economy, and revolutionary socialist. His best-known works are the 1848 pamphlet The Communist Manifesto and the three-volume Das Kapital (1867–1894); the latter employs his theory of historical materialism in an analysis of capitalism, representing his greatest intellectual achievement. His theories and ideas, and their subsequent development, are collectively known as Marxism, and have exerted enormous influence on intellectual, economic, and political history.

Carl Johann Maximovich was a Russian botanist. Maximovich spent most of his life studying the flora of the countries he had visited in the Far East, and naming many new species. He worked at the Saint Petersburg Botanical Gardens from 1852 as curator of the herbarium collection, becoming Director in 1869.

Karl Friedrich May was a German author. He is best known for his 19th century novels of fictitious travels and adventures, set in the American Old West with Winnetou and Old Shatterhand as main protagonists and in the Orient and Middle East with fictional characters Kara Ben Nemsi and Hadschi Halef Omar.

Karl Friedrich Hartmann Mayer was a German jurist and poet of the Swabian school of poets, the circle of Justinus Kerner and the Serach poets' circle under count Alexander von Württemberg (1801–1844). His younger brother Louis Mayer was a landscape painter.