A mortar today is usually a simple, lightweight, man-portable, muzzle-loaded cannon consisting of a smooth-bore (although some models use a rifled barrel) metal tube fixed to a base plate (to spread out the recoil) with a lightweight bipod mount and a sight. Mortars are typically used as indirect fire weapons for close fire support with a variety of ammunition. Historically, mortars were heavy siege artillery. Mortars launch explosive shells (technically called bombs) in high arching ballistic trajectories.

Mortars have been used for hundreds of years. The earliest reported use of mortars was in Korea in a 1413 naval battle when Korean gunsmiths developed the wan'gu (gourd-shaped mortar) (완구, 碗口). The earliest version of the wan'gu dates back to 1407. Ch'oe Hae-san (1380–1443), the son of Ch'oe Mu-sŏn (1325–1395), is generally credited with inventing the wan'gu. In the Ming dynasty, general Qi Jiguang recorded the use of a mini cannon called the hu dun pao that was similar to the mortar.

The first use in siege warfare was at the 1453 siege of Constantinople by Mehmed the Conqueror. An Italian account of the 1456 siege of Belgrade by Giovanni da Tagliacozzo states that the Ottoman Turks used seven mortars that fired "stone shots one Italian mile high". The time of flight of these was apparently long enough that casualties could be avoided by posting observers to give warning of their trajectories.

Early mortars, such as the Pumhart von Steyr, were large and heavy and could not be easily transported. Simply made, these weapons were no more than iron bowls reminiscent of the kitchen and apothecary mortars whence they drew their name. An early transportable mortar was invented by Baron Menno van Coehoorn in 1701. This mortar fired an exploding shell, which had a fuse that was lit by hot gases when fired. The Coehorn mortar gained quick popularity, necessitating a new form of naval ship, the bomb vessel. Mortars played a significant role in the Venetian conquest of Morea, and in the course of this campaign an ammunition depot in the Parthenon was blown up. An early use of these more mobile mortars as field artillery (rather than siege artillery) was by British forces in the suppression of the Jacobite rising of 1719 at the Battle of Glen Shiel. High angle trajectory mortars held a great advantage over standard field guns in the rough terrain of the West Highlands of Scotland.

The mortar had fallen out of general use in Europe by the Napoleonic era, although Manby Mortars were widely used on the coast to launch lines to ships in distress, and interest in their use as a weapon was not revived until the beginning of the 20th century. Mortars were heavily used by both sides during the American Civil War. At the Siege of Vicksburg, General Ulysses S. Grant reported making mortars "by taking logs of the toughest wood that could be found, boring them out for 3 or 5 kg (6 or 12 lb) shells and binding them with strong iron bands. These answered as Coehorns, and shells were successfully thrown from them into the trenches of the enemy".

During the Russo-Japanese War, Lieutenant General Leonid Gobyato of the Imperial Russian Army applied the principles of indirect fire from closed firing positions in the field; and with the collaboration of General Roman Kondratenko, he designed the first mortar that fired navy shells.

Trench mortar was a term used during the World War I to designate few short-range artillery weapons of different sizes. Their ranges were so short that they had to be fired directly from the frontline trenches, hence the name. The German Army studied the Siege of Port Arthur, where heavy artillery had been unable to destroy defensive structures like barbed wire and bunkers. Consequently, they developed a short-barrelled rifled muzzle-loading mortar called the Minenwerfer. Heavily used during World War I, they were made in three sizes: 7.58, 17 and 25 cm (2.98, 6.69 and 9.84 in). The British invented the lightweight Stokes mortar.

It was not until the Stokes mortar was devised by Sir Wilfred Stokes in 1915 during the First World War that the modern mortar transportable by one person was born. In the conditions of trench warfare, there was a great need for a versatile and easily portable weapon that could be manned by troops under cover in the trenches. Stokes' design was initially rejected in June 1915 because it was unable to use existing stocks of British mortar ammunition, and it took the intervention of David Lloyd George (at that time Minister of Munitions) and Lieutenant Colonel J. C. Matheson of the Trench Warfare Supply Department (who reported to Lloyd George) to expedite manufacture of the Stokes mortar. The weapon proved to be extremely useful in the muddy trenches of the Western Front, as a mortar round could be aimed to fall directly into trenches, where artillery shells, because of their low angle of flight, could not possibly go.