He served as pastor of the united churches of Germantown and Philadelphia in 1746–51, organized a synod which met in Philadelphia in 1747, and made extended missionary tours among the German Reformed settlers in Pennsylvania, Maryland, Virginia, New Jersey and New York State.
In Amsterdam, he published (1751) a journal of his experiences and transactions in America, with an account of the Reformed congregations and their dearth of pastors.
As a result of Schlatter's appeal, £20,000 was raised in England and Holland for the establishment of free schools among the Germans in America.
[2] In March 1752, Schlatter returned to Philadelphia with the money and preachers, and in 1755 withdrew from his pastoral activities to become superintendent of the establishment of the schools among the Germans under the auspices of the Society for the Propagation of the Knowledge of God among the Germans, a group of people from London,[2] of whom William Smith was secretary.
He then accepted an offer by Lord Loudoun to become chaplain of the Royal American Regiment, which he accompanied on an expedition to Louisburg.