During the Emin Pasha Relief Expedition Henry Morton Stanley visited Mackay at the Usambiro mission for a short period where he received aid and local information from him.

Mackay worked with David Livingstone and Sir John Kirk to help bring an end to the brutal Arab slave trade in central Africa.

[1][2][3] Mackay's father was also a member of the Royal Geographical Society and an author whose works included a manual on geography, published in 1861, and those in other related fields.

[6] For three years Mackay was set on pursuing engineering, applied mechanics mathematics, and natural philosophy at the university, while his father, however, still did all he could to expand his son's knowledge and involvement in Christianity.

"On 1 November 1873 Mackay continued his education in Germany, in order to study its language in greater depth, feeling that was the first step to becoming acquainted with the abundance of folklore and the customs of that country.

While in Germany Mackay became good friends with one of the ministers of the Berlin Cathedral, whom he regarded as a "genuine Christian and man of wide culture", and was always made welcomed at his home.

During this time was introduced to some of the leading Christians of Berlin, including persons of great prominence in the church society, events of which he often recorded in his diary.

Beginning on 27 April 1876, he sailed aboard the S.S. Peshawur from Southampton, England, and arrived on the island of Zanzibar on 30 May, some fifty miles off the east coast of Tanzania.

At Mpwapwa they collected the necessary equipment, and made preliminary plans for their eight-hundred mile trek across Tanzania to the south end of Lake Victoria.

After Mackay recovered he organized a labor party and began outfitting a caravan of about seventy loads, and placed it in charge of an Englishman named Morton.

In one-hundred days Mackay built a 230 mile wagon road to from Sadani on the coast to Mpwapwa, having to deal with many difficulties through the dense jungle and brush.

[24][25] Mackay's technical skills, especially with metal working, amazed many of the natives, who began to ascribe to him mystical powers, sometimes asking him to perform tasks which he could not accomplish.

In May he subsequently ordered a general massacre of the Christians, known as the Uganda Martyrs, with many deaths effected as the result of being speared, tortured and roasted alive.

Mackay and Ashe, were concerned that the Bishop's arrival arose the worst suspicions of the king and his chiefs, and that it likely would result in bloodshed.

[31] As Mackay had worked with Livingstone, Sir John Kirk[d] and Mutesa in bringing an end to the Arab slave trade near Lake Nyanza.

[36] When Mwanga's plot to kill Christian and Mohammedan soldiers with starvation by marooning them on a small island on the Lake was discovered he was promptly driven from his throne by the combined effort of those soldiers [37] During the subsequent revolt in the autumn of 1888 Mwanga was driven from his throne and fled for his life with his family and others Mwanga'is successor, Kiweewa of Buganda, regarded the Christians with suspicion.

[3][35] The revolt of Muhammad Ahmad that began in 1881 had cut off Equatoria from the outside world, forcing the Governor, Emin Pasha of Egypt to flee south to Uganda to avoid capture.

Here he corresponded with Mackay and others in an effort to procure aid, which ultimately brought the explorer Henry Morton Stanley and the Emin Pasha Relief Expedition to his rescue.

During the expedition Stanley met Mackay in 1889 for the first time at the Usambiro mission station when the former was escorting Emin Pasha, the besieged Egyptian governor of Equatoria.

Mackay had also corresponded with Emin Pasha, and encouraged him to write letters to Dr. Robert Felkin,[f] Consul-General, and Frederick Holmwood a medical missionary in Zanzibar, if he thought it best to get England's support.

[j] A coffin was promptly made using some of the wood Mackay had prepared for the boat, and at two o'clock on the following Sunday Mr. Deekes buried him by the shores of Lake Nyanza (Victoria) which he had always admired.

[50][51] Despite personal losses and other hardships, he had succeeded in establishing a church that did not diminish during the times of persecution and other tribal unrest, and which continued to grow after his departure.

[53][54] Stanley, who had long admired Mackay and his persistent and difficult missionary efforts in Uganda, had often said as much in his letters when he was reporting back to England.