Weni began his career under Teti, and served as a general under Pepi I Meryre and as governor of Upper Egypt during the reign of Merenre Nemtyemsaf I. As judge he investigated the queen who was apparently suspected of involvement in a conspiracy. While he was general, he reorganized the military into a format that was still in use in the New Kingdom.
Weni rose through the ranks of the military to become commander in chief of the army. He was considered by both his contemporaries and many Egyptologists to have been a brilliant tactician. His victories earned him the privilege of being shown leading the troops into battle, a right usually reserved for pharaohs. Weni is the first person, other than a pharaoh, known to have been portrayed in this manner. Many of his battles were in the Levant and the Sinai. He is said to have pursued a group of Bedouins all the way to Mount Carmel. He battled a Bedouin people known as the sand-dwellers at least five times.
While he was commander in chief of the army, he made several key reforms to the military. He began training his troops to have a pre-emptive rather than a defensive posture. Weni included Nubian mercenaries in the army for the first time and he reorganized the army to control infighting amongst the troops and to minimise uncontrolled pillaging. He recorded his reorganization of the army in great detail and his reforms lasted until the time of the New Kingdom.