Mustard seeds have been found in the ruins of communities from the Stone Age, and there are at least 40 species of the plants. Some of the more-used kinds used for condiments are black mustard, hailing from Asia Minor and the Middle East; brown Indian mustard, a Himalayan plant; and white/yellow mustard, hailing from the Mediterranean basin(1). Some claim that Ancient Egypt was the initial source of mustard(2) - archaeological evidence seems to to indicate it was used to flavor food there and King Tut's tomb definitely had mustard seeds in it. Sumerians of ancient Iraq made a paste of mustard seed and put it in verjus, the very acidic juice of unripe grapes. Farming mustard seeds helped bolster prosperity on the Sumerian urban scene(1).
The Greeks used mustard seeds as a pretty broad-spectrum medicine - for such things as snakebite, hysteria, and the plague. Indeed, in the modern world, we've found that they contain powerful nutrients like anti-inflammatory minerals "manganese, iron, magnesium, and selenium" which may help cut down on symptoms of such conditions as arthritis and asthma. The Vitamin B3 and niacin help lower cholesterol and, among other things, help digestion. Possibly early as the 500s BC, mustard seed was used for scorpion stings by Pythagoras; Hippocrates(1) [of the medical oath] "used mustard in medicines and poultices" in the following century. Plasters of mustard were used, among several uses, for toothaches(1).
The Romans put ground mustard seed into wine and introduced mustard to Spain(1) and the north of France, where monks began cultivating it. Monasteries actually made rather hefty profits from mustard by the 800s. The word "mustard" is actually thought to come from grape must, the unfermented grape juice it was mixed with so commonly(2). An official Vatican position for a mustard maker was made by a mustard-enamored pope(1,2). Explorer Vasco de Gama brought mustard to India(1).
References:
1. Sherman, Carol. "Mustard: history of the yellow seed." Hektoen International: A Journal of Medical Humanities. Hektoen International. 18 Feb. 2020. Web. 1 Sep. 2021. https://hekint.org/2020/02/18/mustard-history-of-the-yellow-seed/#.
2. "Home/About us/." G. S. Dunn Dry Mustard Millers. G. S. Dunn Dry Mustard Millers. Web. 1 Sep. 2021. http://www.gsdunn.com/english/?page_id=2943.