Leopold was a younger son and was unlikely to inherit titles or land, so he entered the Army in the service of the Austrian Empire. At one point Leopold was considered as a potential husband for Isabel II of Spain. This did not, however, prove to be realistic, given the resistance of France and other European powers. Spain had been the scene of Great Power rivalry throughout the period since 1815 and all the Great Powers were seeking to exert their influence by supporting different candidates. The Saxe-Coburg family was perceived to be too closely linked with British interests. The Coburg influence extended widely. In Great Britain, Queen Victoria and her husband Prince Albert were Leopold's first cousins, while Leopold I of Belgium was Leopold's paternal and Queen Victoria's maternal uncle, and Leopold's brother was King Ferdinand II of Portugal, husband of Queen Maria II of Portugal. Leopold's candidature in the affair of the Spanish Marriages was used by France as the excuse to negotiate the hasty marriage between Queen Isabella II of Spain and her cousin Francis of Spain, at the same time as that between the son of Louis Philippe and the Queen of Spain's younger sister.
Later, Leopold met Constanze Geiger, a common woman. She bore him a son, called Franz in Vienna on 12 October 1860. Leopold and Constanze married six months later, on 23 April 1861. Immediately, he recognized his son, who was created Freiherr von Ruttenstein on 24 July 1862. The same day, Constanze was created Freifrau von Ruttenstein in her own right. Because the marriage of his parents was unequal (and thus morganatic), Franz was barred from the succession of Koháry and Saxe-Coburg-Gotha. Regardless, he died childless and unmarried on 29 August 1899.