Mustakallio's parents were chaplain Henrik Schwartzberg ja Laura Katariina Stenbäck.

During the last few years, the Finnish missionaries had been criticized heavily in Finland, for an apparent lack of results.

Mustakallio left for Africa on 31 January 1900, and he arrived there on 22 June the same year, together with Emil Liljeblad, who now began his first term in Ovamboland.

He was due to stay for four months on the field, but the visit was shortened to six weeks, one of the reasons for this was that he fell ill with malaria.

In Ovamboland the society was already in a difficult situation, as its work was confined to the kingdom of Ondonga with a population of only some 21000 persons.

When the Boxer Rebellion and its aftermath were over, the first Finnish missionaries to China were sent along, and they stepped ashore in Shanghai in October 1901.

This was due to his intensive approach towards his duties, along with his impatient nature, as he wanted to make quick decisions and achieve more or less instant closures.

He stated that his nervous system had been overburdened and his energy was insufficient for the work as the mission director, and in February 1911 he informed the board of trustees that he had sought a job as the dean of Porvoo.

Around this time, a new inspection trip to Ovamboland was planned, and due to Mustakallio's situation, the vice director Hannu Haahti was sent there.

Mustakallio now felt that he had been excluded from some of his most important duties and protested, but the board did not change their decision.