In compounds, manganese assumes a number of different oxidation states. It is easily raised to the +2 state, for example, by reaction with hydrochloric acid to form manganous chloride, MnCl₂. Manganese is also found in the +3 (manganic) state, but this state is unstable and usually reverts to the +2 state. Both manganous and manganic ions form acidic solutions. Manganese is found in the +4 state largely in manganese dioxide, MnO₂ ; the +4 oxidation state is amphoteric, i.e., in the +4 state manganese can either donate or accept electrons in chemical reactions. Manganese also exists in +6 and +7 states; the +6 state is found in the manganate ion (MnO₄⁻⁻) and the +7 state in the permanganate ion (MnO₄⁻). These ions are stable in basic solutions. There is also evidence for a +1 state (in a complex cyanide) and for an unstable +5 state (in basic solutions). Manganese is found in abundance in nature.

Pyrolusite (MnO₂) is the major ore. Manganese ores are produced principally in the countries of the former Soviet Union, India, the Union of South Africa, Ghana, and Morocco, and to a lesser extent in the United States. The metal is prepared commercially by reduction of its ores with aluminum or, with high purity, by electrolysis of a manganese sulfate solution. Manganese is very important in the steel industry, where it is used as a deoxidizing and desulfurizing agent; no substitute has been found. It is also used in large amounts to toughen and harden steel without making it brittle; it is usually added as ferromanganese. Any steel having between 10% and 15% manganese is known as manganese steel, although almost all steel contains some manganese. Manganese is widely used in making alloys. Manganese bronze and manganese brass are alloys containing manganese, copper, tin, zinc, and small amounts of other metals in varying proportions. Certain alloys containing manganese, aluminum, antimony, and small amounts of copper are highly magnetic.

Compounds of manganese are widely used in industry. Manganese dioxide is used as a drying agent; it catalyzes the oxidation of oils in paints and varnishes. It is also used in the dry cell and to remove the green color caused by iron impurities in glass. Potassium permanganate (KMnO₄) is a powerful oxidizing agent used industrially for bleaching and in chemistry as an analytical reagent. Other compounds find use in glassmaking, as pigments, and as fertilizers. Manganese is needed as a nutrient in small amounts by many plants and animals and by humans. The purple color of amethyst is due to manganese. The element was first isolated in 1774 by J. G. Gahn, although its existence was previously recognized by T. O. Bergman and by K. W. Scheele.