In his boyhood and early youth Alexander frequently visited Saint Petersburg, and he accompanied his uncle, the Tsar, who was much attached to him, during the Bulgarian campaign of 1877. When, under the Treaty of Berlin (1878), Bulgaria became an autonomous principality under the suzerainty of the Ottoman Empire, the Tsar recommended his nephew to the Bulgarians as a candidate for the newly created throne, and the Grand National Assembly unanimously elected Prince Alexander as Prince of Bulgaria (29 April 1879). At that time he held a commission as a lieutenant in the Prussian life-guards at Potsdam. Before proceeding to Bulgaria, Prince Alexander paid visits to the Tsar at Livadia, to the courts of the great powers and to the sultan; a Russian warship then conveyed him to Varna, and after taking the oath to the new constitution at Turnovo (8 July 1879) he went to Sofia. The people everywhere en route greeted him with immense enthusiasm.

The new ruling prince had not had any previous training in governing, and a range of problems confronted him. He found himself caught between the official representatives of Russia, who wanted him to behave as a roi fainéant, and the Bulgarian politicians, who actively pursued their own quarrels with a violence that threatened the stability of Bulgaria.

After attempting to govern under these conditions for nearly two years, the prince, with the consent of the Russian tsar, Alexander assumed absolute power, having suspended the Constitution (9 May 1881). A specially convened assembly voted (13 July 1881) for suspension of the ultra-democratic constitution for a period of seven years. The experiment, however, proved unsuccessful; the monarchical coup infuriated Bulgarian Liberal and Radical politicians, and the real power passed to two Russian generals, Sobolev and Kaulbars, specially despatched from Saint Petersburg. The prince, after vainly endeavouring to obtain the recall of the generals, restored the constitution with the concurrence of all the Bulgarian political parties (19 September 1883). A serious breach with Russia followed, and the part which the prince subsequently played in encouraging the national aspirations of the Bulgarians widened that breach.

The revolution of Plovdiv (18 September 1885), which brought about the union of Eastern Rumelia with Bulgaria, took place with Alexander's consent, and he at once assumed the government of the province. In the year which followed, the prince gave evidence of considerable military and diplomatic ability. He rallied the Bulgarian army, now deprived of its Russian officers, to resist the Serbian invasion, and after a victory at Slivnitza (19 November), which Alexander had little to do with, having arrived in Slivnitsa after the battle (incidentally initiated by a volunteer of the rank of private) was already over, pursued King Milan of Serbia into Serbian territory as far as Pirot, which he captured (27 November). Although the intervention of Austria protected Serbia from the consequences of defeat, Prince Alexander's success sealed the union with Eastern Rumelia, and after long negotiations the sultan Abdul Hamid II nominated the Prince of Bulgaria as governor-general of that province for five years (5 April 1886).

This arrangement, however, cost Alexander much of his popularity in Bulgaria, while discontent prevailed among a number of his officers, who considered themselves slighted in the distribution of rewards at the close of the campaign. A military plot formed, and on the night of 20 August 1886 the conspirators seized the prince in the palace at Sofia and compelled to sign his abdication; they then hurried him to the Danube at Rakhovo, transported him on his yacht to Reni, and handed him over to the Russian authorities, who allowed him to proceed to Lemberg. However, he soon returned to Bulgaria as a result of the success of the counter-revolution led by Stefan Stambolov, which overthrew the provisional government set up by the Russian party at Sofia. His position, however, had become untenable, partly as a result of an ill-considered telegram which he addressed to Tsar Alexander III of Russia on his return. The attitude of Bismarck, who, in conjunction with the Russian and Austrian governments, forbade him to punish the leaders of the military conspiracy, also undermined Alexander's position. He therefore issued a manifesto resigning the throne, and left Bulgaria on 8 September 1886.

Alexander then retired into private life. A few years later he married Johanna Loisinger, an actress, and assumed the style of Count von Hartenau (6 February 1889). There were a son and a daughter from this marriage. The last years of his life he spent principally at Graz, where he held a local command in the Austrian army, and where he died on 23 October 1893. His remains, brought to Sofia, received a public funeral there, and were buried in a mausoleum erected to his memory.

Prince Alexander possessed much charm and amiability of manner; he was tall, dignified and strikingly handsome. Competent authorities have generally recognised his capabilities as a soldier. As a ruler he committed some errors, but his youth and inexperience and the extreme difficulty of his position account for much. He had some aptitude for diplomacy, and his intuitive insight and perception of character sometimes enabled him to outwit the crafty politicians who surrounded him. His principal fault remained a want of tenacity and resolution; his tendency to unguarded language undoubtedly increased the number of his enemies.

Battenberg Hill on Livingston Island in the South Shetland Islands, Antarctica, is named after Prince Alexander Battenberg of Bulgaria.