The Tainui iwi share a common ancestry from Polynesian migrants who arrived in New Zealand on the Tainui waka, which voyaged across the Pacific Ocean from Hawaiki to Aotearoa (North Island) approximately 800 years ago. According to Pei Te Hurinui Jones, the Tainui historian, Tainui first entered the Waikato about 1400 bringing with them kumara plants. By about 1450 they had conquered the last of the indigenous people in a battle at Atiamuri.
During the late 1840s and early 1850s European missionaries introduced Tainui to modern inventions such as the water mill and gave then instruction in how to raise various European crops: potatoes were particularly widely planted. They set up a trade school in Te Awamutu to educate young Tainui so they became literate and taught the basics of numeracy and farming skills. Two mills were built to grind the wheat into flour – one near Cambridge on a stream leading to the Waikato River. Some parts of the mill are still visible. Later in the 1850s six others were built in the general area. Produce was exported as far as Victoria and California.
However, the relationship was far from one-sided. The Tainui tribe provided food to the European settlers, and "the present European population…would have been literally starved out of the country but for the extraordinary exertions made by the aboriginal inhabitants to supply them with cheap provisions", as the Southern Cross newspaper reported in 1844. A year later, when the less than 4,000 settlers of Auckland were under threat from an attack from Ngāpuhi from the south, Tainui rangatira Te Wherowhero, asked for assistance in the planned attack, responded “You must fight me if you come on to Auckland; for these Europeans are under my protection,” - referring to Auckland as the "hem of his cloak" and placing it under his personal tapu.
During this time large numbers of new migrants came to Auckland and Te Wherowhero established a house in Mangere so he could oversee trade and get advice from the government. For a brief period until the mid-1850s Tainui made a good return from selling food to the new settlers but this all came to a sudden end when traders realised they could get food – especially flour – much cheaper from New South Wales. Tainui set up a bank at Cambridge to take the deposits of Maori traders but this was burnt down by the people when it was found that chiefs were using the money as their own.
Relationships between Europeans and Tainui soured as Europeans began to outnumber Māori (around 1858, across all of New Zealand), stopping them from being dependant on friendly tribes for food and protection. At the same time as respect for even high-ranking Māori waned, desire for land settled by them grew among Europeans. At the outbreak of the First Taranaki War, "friendly Māori" in Auckland had to be issued with arm badges to protect them from assault.
Tainui people were expelled from the Auckland area in 1863 because of their refusal to take the oath of allegiance to the Crown and hand in their weapons which the governor thought posed a threat to Auckland and the new settlers as it had done in Taranaki.
Tainui were the tribe responsible for the setting up of the Kīngitanga in 1858 – a pan-Māori movement of mainly central North Island iwi who aimed at establishing a separate Māori nation with a Māori King. The key aim was the refusal of the kingites to sell land to the government. The first Māori king was the great Waikato warrior Te Wherowhero who came from a great line of rangatira. Tainui, who had conquered much Taranaki land, sent warriors to help fight the settlers and British soldiers in Taranaki to prevent minor chiefs selling land to the government. Missionaries at Te Awamutu told the Kīngitanga they would be considered rebels by the government after they refused to take an oath of allegiance to the Crown. Te Awamutu was a missionary settlement built by the missionaries and Māori Christians in July 1839 after they observed Tainui cannibals who had been fighting at Rotorua, return with 60 backpacks of human remains and proceed to cook and eat them in the Otawhao pa.