In the Early Dynastic and Old Kingdom period each Nome or tribe would raise its own troops, based on a short term of active service where young men would serve for a set period upon coming of age. They would then be subject to being mustered whenever trained men were needed. The army of the Pharaoh in this period would be made up almost completely of militia contingents from the Nomes who were loyal to the Pharaoh. The Pharaoh also had a small standing force or royal guard, which would serve as the core of the armies.
The Army during this period was formed totally of infantry made up of heavy and light infantry, and skirmishers. The army was commanded by the Pharaoh or the Vizier when it took to the field and marched to war.
Mercenaries were recruited from Nubia, and in the later period from Libya, when it was felt necessary. In the later Old Kingdom armies, mercenaries seem to have made up the majority of the troops in the Pharaoh's armies. This was probably because the Pharaoh doubted the loyalty of the troops supplied by the Nomes.
The Middle Kingdom established a standing army to avoid the problems of the Old Kingdom's reliance on part time troops that were often more loyal to their Nomes than to the Pharaoh. To encourage loyalty to the Pharaoh and create a feeling of esprit de corps, the Pharaoh would award gallantry and loyal service with rich gifts of gold, jewel encrusted weapons and gold collars and armbands.
The numbers of mercenaries recruited in the Middle Kingdom Armies were in much smaller numbers than in earlier armies. They were still mostly from Nubia and Libya, but they were kept to being only a minor part of the field armies and were instead used as garrisons and for police work. This both freed up Egyptian troops for the field army and also helped to keep the governors of the Nomes in-line.
The Armies of the Middle Kingdom remained an all-infantry force, as they would until the adoption of the chariot from the Hyksôs sometime between 1600BC and 1500BC.