Leul Sahle Maryam was born in Angolela, near Debre Birhan, Shewa. Sahle Maryam was the son of Negus Haile Melekot of Shewa and Woizero Ijigayehu. Haile Melekot and Ejigayehu were married in 1844 and divorced in 1845. Prior to his death in 1855, Negus Haile Melekot named Sahle Maryam as successor to the throne of Shewa. Shortly after Haile Melekot died, Sahle Maryam was taken prisoner by Nəgusä Nägäst Tewodros II. Tewodros was a former minor noble originally named Kassa of Qwara. Following Nəgusä Nägäst Tewodros II's conquest of Shewa, he had young Sahle Maryam transferred to his mountain stronghold of Magdala. Still, Tewodros treated Sahle Maryam well. He even offered him the hand of his daughter Altash Tewodros in marriage, which Sahle Maryam accepted. Upon Sahle Maryam's imprisonment, his uncle, Haile Mikael was appointed as Shum of Shewa by Nəgusä Nägäst Tewodros II with the title of Meridazmach. However, Meridazmach Haile Mikael rebelled against Tewodros, resulting in his being replaced by the non-royal Ato Bezabeh as Shum. However, Ato Bezabeh in turn then rebelled against the Emperor and proclaimed himself Negus of Shewa. Although the Shewan royals imprisoned at Magdala had been largely complacent as long as a member of their family ruled over Shewa, this usurpation by a commoner was not palatable to them. They plotted the escape of Sahle Maryam from Magdala; with the help of Mohammed Ali and Queen Worqitu of Wollo, he escaped from Magdala the night of 1 July 1865, abandoning his wife, and returned to Shewa. Enraged, Emperor Tewodros slaughtered 29 Oromo hostages then had 12 Amhara notables beaten to death with bamboo rods.

Bezabeh's attempt to raise an army against Sahle Maryam failed miserably; thousands of Shewans rallied to the flag of the son of Negus Haile Melekot and even Bezabeh's own soldiers deserted him for the returning prince. Leul Sahle Maryam entered Ankober and proclaimed himself Negus with the name of Menelik. While Negus Menelik reclaimed his ancestral crown, he also made a claim on the Imperial throne, as a direct descendant male line of Nəgusä Nägäst Lebna Dengel. However, he made no overt attempt to assert this claim during this time; Marcus interprets his lack of decisive action not only to Menelik's lack of confidence and experience, but that "he was emotionally incapable of helping to destroy the man who had treated him as a son." By failing to take part in the 1868 Expedition to Abyssinia, he allowed his rival Kassai to benefit with gifts of modern weapons and supplies from the British. Afterwards other challenges — a revolt amongst the Wollo to the north, the intrigues of his next wife Baffana to replace him with her choice of ruler, military failures against the Arsi Oromo to the south east — kept Menelik from directly confronting Kassai until after his rival had brought an Abuna from Egypt who crowned him Nəgusä Nägäst Yohannes IV. Eventually Menelik acquiesced to the superior position of Yohannes and, on 20 March 1878, Menelik "approached Yohannes on foot. He was carrying a rock on his neck and his face was down in the traditional form of submission. However, very aware of how precarious his own position was, Yohannes recognized Menelik as Negus of Shewa and gave him numerous presents which included four cannons, several hundred modern Remington rifles, and ammunition for both.

On 10 March 1889, Emperor Yohannes was killed in a war against the dervishes during the Battle of Gallabat (Matemma). With his dying breaths, Yohannes declared his natural son, Dejazmach Mengesha Yohannes, as his heir. On 25 March, upon hearing of the death of Yohannes, Negus Menelik immediately proclaimed himself as Nəgusä Nägäst. The succession now lay between Mengesha Yohannes of Tigray and Menelik of Shewa. Menelik argued that while the family of Yohannes IV claimed descent from King Solomon and the Queen of Sheba through females of the dynasty, his own claim was based on uninterrupted direct male lineage which made the claims of the House of Shewa equal to those of the elder Gondar line of the dynasty. In the end, Menelik was able to obtain the allegiance of a large majority of the Ethiopian nobility. On 3 November 1889, Menelik was consecrated and crowned as Nəgusä Nägäst before a glittering crowd of dignitaries and clegy. He was crowned by Abuna Mattewos, Bishop of Shewa, at the Church of Mary on Mount Entoto. The newly consecrated and crowned Nəgusä Nägäst Menelik II quickly toured the north in force. He received the submission of the local officials in Lasta, Yejju, Gojjam, Welo, and Begemder. Menelik, and later his daughter Zauditu, would be the last Ethiopian monarchs who could claim uninterrupted direct male descent from King Solomon and the Queen of Sheba (both Lij Iyasu and Emperor Haile Selassie were in the female line, Iyasu through his mother Shewarega Menelik, and Haile Selassie through his paternal grandmother, Tenagnework Sahle Selassie).

In April 1889, while claiming the throne against Mengesha Yohannes, Menelik signed at Wuchale in Wollo province (Uccialli in Italian) a treaty with Italy, acknowledging the establishment of the new Italian Colony of Eritrea with its seat at Asmara. This colony had previously been a Republic of Several Districts under the Title of Medri-Bahri (Land of the Sea), it consisted of Hamasien, Akele-Guzay, Seraye. In addition, it was referred to as Merab Melash meaning the "Land Beyond the River". The river was the political boundary that separated Medri-Bahri (Ras Woldeinkel, Ruler of Medri Bahri) and Tigrai (Ras Mengesha). Upon the treaty with Italy, Emperor Menelik II in 1889 stated

The territories north of the Merab Melash (Modern Eritrea) do not belong to nor are under my rule. I am the Emperor of Abysinnia. The lands referred to as Eritrea is not peopled by Abysinnians, they are Adals, Bejas, and Tigres. Abysinnia will defend her territories but it will not fight for foreign lands of which Eritrea is to my knowledge.

However, it was soon found that the Italian version of one of the articles of the treaty placed the Ethiopean Empire under an Italian protectorate, while the Amharic version did not. Emperor Menelik denounced it and demanded that the Italian version be changed. Negotiations failed, so Menelik renounced the treaty, leading Italy to declare war and invade from Eritrea. After defeating the Italians at Amba Alagi and Mekele, Menelik inflicted an even greater defeat on them at Adowa on 1 arch 1896, forcing them to capitulate. A treaty was signed at Addis Ababa recognizing the absolute sovereign independence of Ethiopia.

The first diplomatic contacts of the Ethiopian government with Russia were in 1888-1889 with adventurer Ashinov, working under Konstantin Petrovich Pobedonostsev. Menelik II became the sincere Great friend of Russia, and only union with Russia could be a base of policy of integration of Ethiopia 1893-1913, owing to the necessity to counteract the British policy of sabotage of the integration of Ethiopia initiated by the assassination by the British troops of the Tewodros II, (The British 1868 Expedition to Abyssinia, also stole the book "The Kebra Nagast", (but translated only in 1932)). That is why only Russia could become the main strategic ally of Ethiopia during 1893-1913. Nicholai Leontyev became a Russian friend and adviser of Menelik II. In 1893 the meeting between Leontyev and Menelik II set up the foundation for their mutual friendship. Menelik II introduced the title of count, which had never existed in his country before. To the sound of a three-shot rifle salute, Nicholai Leontyev was handed an official document that declared him to be Count Abai. One by one, this Russian man fulfilled Menelik's important errands. From 1893 to 1913, thousands of Russian volunteers visited Ethiopia with the purpose of granting it military and civil help. During the visit of a Russian diplomatic and military mission in 1893, Menelik II concluded a strong alliance with that country. As a result of that alliance, from 1893-1913, Russia sponsored the visits of thousands of advisers and volunteers to Ethiopia. Two friendships that evolved from these visits were friendships between Menelik II and Alexander Bulatovich and also between Menelek II and Nikolay Gumilyov the great poet.

Menelik II's French sympathies were shown in a reported official offer of treasure towards payment of the indemnity at the close of the Franco-Prussian War, and in February 1897 he concluded a commercial treaty with France on very favourable terms. He also gave assistance to French officers who sought to reach the upper Nile from Ethiopia to join forces with the Marchand Mission; and Ethiopian armies were sent towards the Nile, but withdrew when the Fashoda Crisis between France and the United Kingdom cooled off. A British mission under Sir Rennell Rodd in May 1897, however, was cordially received, and Menelik agreed to a settlement of the Somali boundaries, to keep open to British commerce the caravan route between Zaila and Harrar, and to prevent the transit of munitions of war to the Mahdists, whom he proclaimed enemies of Ethiopia. In the following year the Sudan was reconquered by an Anglo-Egyptian army and thereafter cordial relations between Menelik and the British authorities were established. In 1889 and subsequent years, Menelik sent forces to co-operate with the British troops engaged against a Somali leader, Sayyid Mohammed Abdullah Hassan.

Menelik had in 1898 crushed a rebellion by Ras Mengesha Yohannes (who died in 1906). He directed his efforts thenceforth to the consolidation of his authority, and in a certain degree, to the opening up of his country to western civilization. Menelik’s clemency to Ras Mangasha, whom he compelled to submit and then made hereditary Prince of his native Tigray, was ill repaid by a long series of revolts by that prince. Menelik focused much of his energy on development and modernization of his country after this threat to his throne was firmly ended. He had granted in 1894 a concession for the building of a railway to his capital from the French port of Djibouti, but, alarmed by a claim made by France in 1902 to the control of the line in Ethiopian territory, he stopped for four years the extension of the railway beyond Dire Dawa. When in 1906 France, the United Kingdom and Italy came to an agreement on the subject, granting control to a joint venture corporation, Menelik officially reiterated his full sovereign rights over the whole of his empire.

In May 1909, the emperor’s grandson Lij Iyasu (later Iyasu V) by his late daughter Shoaregga, then a lad of thirteen, was married to Romanework Mengesha, granddaughter of the Emperor Yohannes IV by his natural son Ras Mengesha, and was also the niece of Empress Taytu. Two days later Iyasu was publicly proclaimed at Addis Ababa as Menelik’s successor. At that time the emperor was seriously ill and as his ill-health continued, a council of regency — from which the empress was excluded — was formed in March 1910. Lij Iyasu's marriage to Romanework Mengesha was dissolved, and he married Seble Wongel Hailu, daughter of Ras Hailu, and granddaughter of Negus Tekle Haymanot of Gojjam. On 12 December 1913 Emperor Menelek II died of a stroke and was buried secretly at the Se'el Bet Kidane Meheret Church on the grounds of the Imperial Palace. Official news of his death was kept from the public for several years by order of Lij Iyasu, although it was soon widely known. Following the deposition of Lij Iyasu in 1916, and the crowning of Menelik's daughter Zauditu as Empress of Ethiopia, Menelik II was reburied in the specially built church at Ba'eta Le Mariam Monastery of Addis Ababa.

Menelik II was fascinated by modernity, and like Tewodros II before him, had a keen ambition to introduce Western technological and administrative advances into Ethiopia. Following the rush by the major powers to establish diplomatic relations following the Ethiopian victory at Adowa, more and more westerners began to travel to Ethiopia looking for trade, farming, hunting and mineral exploration concessions. Menelik II founded the first modern bank in Ethiopia, the Bank of Abyssinia, introduced the first modern postal system, signed the agreement and initiated work that established the Addis Ababa-Djibouti railway with the French, introduced electricity to Addis Ababa, as well as the telephone, telegraph, the motor car and modern plumbing. He attempted unsuccessfully to introduce coinage to replace the Maria Theresa thaler.

During the 1890s, Menelik heard about the modern method of executing criminals using electric chairs, and he ordered 3 for his kingdom. When the chairs arrived, Menelik learnt they would not work, as Ethiopia did not yet have an electrical power industry. Rather than waste his investment, Menelik used one of the chairs as his throne, sending another to his "second" (Lique Mekwas) Abate Ba-Yalew.

During a particularly devastating famine caused by Rinderpest early in his reign, Menelik personally went out with a hand-held hoe to furrow the fields to show that there was no shame in plowing fields by hand without oxen, something Ethiopian highlanders had been too proud to consider previously. He also forgave taxes during this particularly severe famine.

Later in his reign, Menelik established the first Cabinet of Ministers to help in the administration of the Empire, appointing trusted and widely respected nobles and retainers to the first Ministries. These ministers would remain in place long after his death, serving in their posts through the brief reign of Lij Iyasu and into the reign of Empress Zauditu. They played a key role in deposing Lij Iyasu.

In 1864, Menelik married Altash Tewodros who he divorced in 1865. The marriage produced no children. In 1865, he married Befana Gatchew who he divorced in 1882. The marriage produced no children. Finally, in 1883, he married Taytu Betul who remained his wife until his death. From 1906, for all intents and purposes, Taytu Betul ruled in Menelik's stead during his infirmity. Leult Altash Tewodros was a noblewoman of Imperial blood and she was the daughter of Nəgusä Nägäst Tewodros II. She and Menelik were married during the time that Menelik was held captive by Tewodros. The marriage ended when Menelik escaped captivity. Befana Gatchew was married to Menelik for seventeen years from 1865 to 1882. Her brother was Zeka Gatchew, the first husband of Taytu Betul. Leult Taytu Betul was a noblewoman of Imperial blood and a member of one of the leading families of the regions of Semien, Yejju in modern Wollo, and Begemder. Her paternal uncle, Dejazmatch Wube Haile Maryam of Semien, had been the ruler of Tigray and much of northern Ethiopia. She had been married four times previously and exercised considerable influence. Menelik and Taytu would have no children. Previous to his marriage to Taytu Betul, Menelik fathered several "natural" children. Three natural children that Menelik recognized were Leult Shoagarad Menelik, born 1867, Leult Zauditu Menelik, born 1876, and Leul Asfa Wossen Menelik, born 1873. In 1886, Menelik married ten-year-old Leult Zauditu to Ras Araya Selassie Yohannes, the fifteen-year-old son of Nəgusä Nägäst Yohannes. In May 1888, Ras Araya Selassie died. All of Zauditu's marriages were to be childless. In 1892, twenty-five-year-old Leult Shoagarad was married to forty-two-year-old Ras Mikael of Wollo. They had children and Menelik's successor, Lij Iyasu, was the son of Shoagarad. Leul Asfa Wossen died when he was about fifteen-years old. Only Shoagarad has present day descendants. Rumoured natural children of the Emperor include Ras Birru Wolde Gabriel and Dejazmach Kebede Tessema. The latter, in turn, was possibly the natural grandfather of Colonel Mengistu Haile Mariam, the communist leader of the Derg, who eventually deposed the monarchy and assumed power in Ethiopia from 1974 to 1991.

In the early morning hours of 12 December 1913, Nəgusä Nägäst Menelik II died. He was buried quickly without announcement or ceremony. Menelik II is considered by Pan-Africanist scholar Ali Mazrui to be one of the most powerful black men in history.