In Hankou, he treated patients in the London Missionary Society hospital, learned Chinese, and engaged in evangelism.
In Tianjin, MacKenzie constructed and ran a hospital with the help of the Viceroy's patronage and also established the Tientsin Medical School.
After moving to Bristol when he was an infant, John MacKenzie's father became deeply attached to Reverend Matthew Dickie at the Presbyterian Church.
MacKenzie started to regularly attend the meetings of the Young Men's Christian Association in Bristol.
After reading his book, MacKenzie decided to ask his parents if he could quit his job and begin to study medicine with the goal of going to China as a medical missionary.
He faced tremendous prejudice because many of the Chinese did not trust the medicine of the foreign part of the Christians living in Hankou.
In August, MacKenzie suffered heavily from malaria, but recovered in a month and he soon became head doctor at the hospital.
He began his plans to construct a medical school with the hopes of teaching Chinese doctors how to perform surgery.
In December 1876, Dr. MacKenzie travelled to Shanghai to marry Miss Travers, the woman he met while doing Christian work in Bristol.
There was a cholera outbreak that MacKenzie treated and there was also an increase in surgeries, indicating the building trust of the Chinese to western medicine.
MacKenzie used his own money to buy foreign drugs to dispense to people in need but only a few showed up each day.
[1] Even though MacKenzie was struggling to keep up with the hospital, he still had the goal of creating a medical school in China to teach the Chinese of western medicine.
MacKenzie withdrew from being involved in the medical school because he didn't want to spend his time raising pupils up to be Government workers.