Mark was an Old Norse (mörk) and Germanic (mark) unit of measure commonly used for precious metals, especially gold and silver. It was equal to about eight ounces (half a pound). Many nations had coins named some variant of 'mark'.

Not England, though! In England it was never used in the form of a coin; instead, it was used as a unit of accounting. For reasons unclear, the mark (m) was equivalent to two thirds of a pound (£), or 13s 4d. The mark was used in accounting and law (e.g., in 1181 every freeman having goods worth 10 marks was required to have a helmet, mail shirt, and spear). As one pound constituted 240 pence, division by thirds was easy and common; one third of a pound was half a mark, two thirds were one mark, and three thirds were simply a pound.

The mark was introduced to England during the Danelaw, the period following the invasion of the Danish armies in the 800s and lasting to 1066. The use of the mark as an accounting term continued at least until the 1700s. Counting money under the LSD system was simplified in that the coins were scaled to their weight: a pound sterling weighed one troy pound; a penny was one pennyweight of silver. You could quite literally count your money simply by setting it on a scale. How the original half-pound mark came to be used for the two thirds weight, I do not know; it no doubt has something to do with the many various definitions of pound used in various lands throughout the years.

In 1816 the government adopted the gold standard, which messed up the silver accounting system; at the same time, paper notes were becoming more common, and other countries in Europe were adopting decimalization, making accounting in thirds passé. France switched over to decimal currency in 1795, and Austria-Hungary in 1857. Decimalization may not have been a major factor, however, as in Baltic regions the local mark was already well out-of-step with the British mark, being one third of a Reichsthaler, not two thirds. Regardless, the term died out quickly in the early 1800s, and is an obscure footnote today.