Mentuhotep II was born into the Eleventh Dynasty, a line of Theban princes who had gradually expanded their power from the city of Amun up the Nile valley during the chaos of the First Intermediate Period. His father was Intef III, the Theban king who had pushed the northern frontier as far as the Tenth Nome, near the city of Asyut, where the forces of Thebes met the armies of Herakleopolis in a militarised border that divided Egypt in two. His mother was Queen Iah, a woman whose name meant "Moon" and who may have been a priestess of Hathor, the cow goddess of motherhood, music, and joy. The Theban kings claimed descent from a family of local princes, and Mentuhotep II took care to emphasise this lineage in inscriptions at Wadi Shatt el-Rigal and at the temple of Montu in Tod, where he depicted himself alongside the Intef family. Whether this genealogy was genuine or politically convenient, it served to legitimise his rule as the heir of a Theban tradition rather than a usurper from the south.

When Mentuhotep II ascended the throne around 2055 BCE, Egypt had been divided for more than a century. The Old Kingdom, the age of pyramid builders that had lasted from the Third to the Sixth Dynasty, had collapsed under the weight of climate change, administrative decay, and the rise of powerful provincial governors called nomarchs who had stripped the pharaoh of his authority. The Ninth and Tenth Dynasties, based at Herakleopolis in Lower Egypt, controlled the Delta and the northern Nile valley, while the Eleventh Dynasty at Thebes ruled the south. The two kingdoms existed in a state of armed peace, raiding each other's borders and competing for control of the trade routes and the fertile lands between them.

For the first fourteen years of his reign, Mentuhotep II consolidated his hold on Upper Egypt, defeating loyalists to Herakleopolis and absorbing nomadic groups into his territory. Then, around his fourteenth regnal year, the Herakleopolitan kings committed an act that changed the course of the war: they desecrated the royal necropolis of Abydos, the sacred burial ground of the First Dynasty kings who had unified Egypt three thousand years earlier, and the cult centre of Osiris, the murdered god whose resurrection was the foundation of Egyptian belief in eternal life. The desecration was an act of sacrilege so profound that it gave Mentuhotep II both the moral justification and the popular support he needed to launch a war of total conquest. He dispatched his armies north, and after years of campaigning, his forces captured Herakleopolis and destroyed the rival dynasty. By his thirty-ninth regnal year, Egypt was unified once more, and Mentuhotep II changed his Horus name to Sematawy, "He Who Unifies the Two Lands," a title that no pharaoh had borne since the First Dynasty.

The reunification was not merely military. Mentuhotep II reformed the Egyptian government to prevent the decentralisation that had caused the collapse of the Old Kingdom. He stripped the nomarchs of much of their power, replacing provincial governors with loyal appointees from Thebes who answered directly to the crown. He created new governmental posts, sent officials from the capital to tour the provinces regularly, and recentralised the administration, the army, and the priesthoods under royal control. He promoted the cult of Amun, the local god of Thebes, to national prominence, a decision that would have profound consequences two hundred years later when the Theban priesthood of Amun became the wealthiest and most powerful religious institution in Egypt. He also restored trade, sending expeditions to Punt on the Red Sea coast for incense and myrrh, and to Byblos on the Phoenician coast for cedar timber.

The most enduring monument of Mentuhotep II's reign is his mortuary temple at Deir el-Bahri, a site on the west bank of the Nile opposite Thebes, nestled in a natural amphitheatre of cliffs that the Egyptians called geser, "the sacred." The temple, named Akh-sut Nebhepetre, was unlike anything that had come before it. It combined elements of the Old Kingdom pyramid complex - a valley temple on the water's edge, a causeway, and a mortuary temple - with the saff tomb tradition of his Theban predecessors, whose rock-cut tombs were fronted by rows of pillars. The result was a terraced complex that rose in stages from the valley floor to the cliff face, with a central core that may have been a symbolic pyramid or a mastaba-like structure, surrounded by colonnades, courtyards, and gardens planted with sycamore and tamarisk trees. A one-hundred-and-fifty-foot causeway led from the valley temple up to the main courtyard, lined with Osirian statues of the king.

The temple was revolutionary not only in architecture but in theology. It was the earliest mortuary temple in which the king was explicitly identified with Osiris, the god of resurrection. The walls were decorated with scenes of Mentuhotep II smiting enemies, hunting in the marshes, and making offerings to the gods, but the dominant theme was Osirian: the king as the dead god who would rise again. Some reliefs show him with pitch-black skin, wearing the red crown of Lower Egypt and a divine beard, his legs swollen in a manner that some scholars have interpreted as elephantiasis but that may simply be an artistic convention emphasising his connection to the black, fertile silt of the Nile and to Osiris himself. The temple also housed the first known cult of Hathor at Deir el-Bahri, establishing the site's sacred association with the goddess that would endure for a thousand years.

One of the most intriguing features of the temple is the Bab el-Hosan, the "Gate of the Horseman," an underground passage discovered in 1900 when Howard Carter, the archaeologist who would later find the tomb of Tutankhamun, was riding through the temple courtyard and his horse stumbled on uneven ground, revealing a hidden entrance. Carter entered the passage, which extended one hundred and fifty metres into the rock, and found a chamber containing a seated statue of the king wrapped in linen and a wooden coffin that bore no name. The statue, now one of the treasures of the Egyptian Museum in Cairo, shows Mentuhotep II seated and wearing a heb-sed cloak, the garment of the jubilee festival that renewed the king's divine power. The chamber may have been intended as the king's original burial place before the plan was changed, making it an early form of Osirian cenotaph, a symbolic tomb for a god who would not stay dead.

At the back of the temple, behind the central terrace, six shrines were erected for young women who were buried within the complex. All six were identified as "unique royal favourites and priestesses of Hathor," and their ages at death ranged from approximately five to twenty-two years. Their names were Ashayet, Kawit, Henhenet, Kemsit, Sadeh, and one whose name is lost. Each had a limestone shrine decorated with carved reliefs and painted in bold colours, and behind each shrine lay a tomb with a stone sarcophagus. The sarcophagus of Ashayet, now in the Egyptian Museum in Cairo, retains much of its original colour and shows the lady seated and holding a lotus flower to her nose while a scribe reads a liturgy to her. Why these six young women were given the honour of burial in the king's temple remains a mystery. Some scholars suggest they were symbolic companions for the king in the afterlife, human substitutes for the paddle dolls that were traditionally placed in tombs to entertain the deceased. Others believe they may have been members of the royal family or the king's wives, though only Queen Tem, the mother of his successor Mentuhotep III, is known to have held the title of Great Royal Wife.

Mentuhotep II reigned for fifty-one years according to the Turin King List, one of the longest reigns in Egyptian history, and died around 2004 BCE. He was succeeded by his son Mentuhotep III, who continued the dynasty for a brief period before the Twelfth Dynasty, founded by Amenemhat I, moved the capital to Itjtawy near the Faiyum and began the classical phase of the Middle Kingdom. But it was Mentuhotep II's reunification that made the Middle Kingdom possible, and his temple at Deir el-Bahri that provided the architectural model for Hatshepsut's terraced masterpiece five centuries later. The Egyptians remembered him as one of their three great national heroes, alongside Menes, the founder of the First Dynasty, and Ahmose I, the founder of the New Kingdom.