The Mills bomb was the first modern fragmentation grenade.
Earlier grenades used in World War I used a stick attached to the grenade, and often were caught in barbed wire when thrown, often with fatal results to the users. The Germans remained faithful to that type.
William Mills' invention was published as Improvements in, or relating to, grenades or other like apparatus. He described himself as an engineer at the Atlas Aluminium Works, Grove Street, Birmingham. That publication incorporated two filed inventions, 1915/02468 and 1915/03559, and refers to an earlier 1915/02111. That, dated 10 February 1915, had also been published, but the later specification is usually given the credit, although they do look rather similar. The American patent specification, filed in June 1915, was published as US1178092. Here is a page from its drawings.
It was a grooved cast iron “pineapple" with a central striker and close hand lever over it which was secured with a pin. It exploded four seconds after the pin was removed -- seven seconds had been found to be too long, giving time for the enemy to take cover or to even throw it back.
According to Mills' notes the casing was grooved to make it easier to grip, rather than to increase fragmentation. The grooves mean that when the grenade explodes, an increased number of fragments are produced. It was quickly adopted by the British Army as their standard hand grenade, although it was repeatedly modified. There is a Wikipedia article on the Mills bomb.
According to that article there was a patent dispute with a Belgian Captain, Leon Roland. This is presumably based on Roland's patent GB1913/18766, which is illustrated below.
According to Anthony Saunders' 2011 book Reinventing warfare 1914-18: novel munitions and tactics of trench warfare Mills did base his design on Roland's invention, but the crucial difference was that the Mills grenade worked and the Roland one did not.
Besides patenting in the USA and France, the 1915 invention was patented in Germany (as DE339387). It was filed there in September 1919, taking advantage of a dispensation for foreign patents having to be filed within the normally required 12 months of the original filing. The problems of World War I for intellectual property -- such as trying to patent in an enemy country -- were covered by the Berne Arrangement of 1920, and were presumably anticipated by Mills when he filed it.
Mills claimed to have lost money from his invention, which was manufactured by his own factory. He was awarded £27,750 by the government for the invention, and apparently tried (unsuccessfully) to avoid paying income tax on this sum.