La Unión is situated in the Region of Murcia in the southeast of Spain. It had a population of 19,907 at 1 January 2018. It has the elevation of 86 m. It has balmy winters. The sun shines 320 days per year. La Unión is situated in one of the sunniest areas in Europe; this kind climate makes possible the many leisure activities, popular fiestas, sports, and cultural activities that are held in the town. The town is linked by regular train to Cartagena which allows views of the past industrial heritage of the area (lead, alum and silver mining) and the more modern occupations of agriculture and tourism. La Unión lies within the built-up area of Cartagena and is surrounded on all landward sides by the City of Cartagena.

The rich mines of La Union provided most of the silver and lead needed by the Late Roman Republic, as it was studied by the archaeologist Eulalia Sintas Martínez.