After returning from her first furlough in 1880, Mary  was sent to Old Town in Calabar a few miles from Duke Town. From then on she proceeded to work in increasingly remote villages up river, setting up new missions and building churches and schools. As the only missionary in these areas, she was helped by the local people to build mission houses but often lived in very basic mud huts sleeping on the floor surrounded by her children and others she had rescued and their mothers.

One of the ways in which Mary tried to improve the lives of the villagers in the communities in which she lived and worked, was to encourage the tribes to trade with each other instead of fighting. Her skill in speaking  Efik, which was the main trading language, meant that she could converse easily with local leaders and discuss the benefits of trade.

She contacted King Eyo VII in Creek Town and asked him to invite a delegation of tribesmen from Okoyong to come and discuss the possibility of trading their palm oil and other produce. Considerable discussion was needed to persuade the Okoyong Chiefs to leave their weapons behind before embarking on the visit, which they eventually agreed to as long as Mary came along as well.

Chiefs became aware of the benefit of their people being able to speak  English to trade more widely  and so numbers in her classes increased to learn ‘Book’ as they called the Bible teaching. This was not necessarily to be able to learn the Bible, but to be able to negotiate with other traders from other countries in Calabar port.

As word of ‘The White Ma’ spread around the Calabar district as the years went on,  Mary was called on to help sort out local disputes in many difficult situations. Living with the people she was able to understand the detailed intricacies and histories behind local situations and disagreements. She was asked to attend court hearings for the local tribal justice systems and eventually oversee them and give judgments. Her skill in healthcare and being able to treat a variety of ailments, also enabled her name to become well known and trusted around the region and she would be sought out to give advice and medicine.

Calabar or Southern Nigeria as we now know it, became a British Protectorate in 1900.  European countries such as Britain and France were gradually creating more formalized systems in West Africa following a history of trade and colonization. This brought some benefits such as new roads, but many problems.

Consul Claud Macdonald who arrived in Calabar in 1891 planned to set up a new system of courts in the area as part of the Protectorate. He had heard about the work of Mary Slessor and asked her about the planned court system. In her usual forthright manner she said that external people trying to settle local disputes would be a disaster. She therefore found herself appointed as his vice-consul to Okoyong appointed by the Secretary of State. She then had to manage challenging situations where she was representing the local people and dealing with British government officials, all of this, while continuing her missionary work and raising her children.

This role did however, make her the first female magistrate in the British Empire.