Jacob E. Mallmann was born in Brooklyn, New York. He graduated from Princeton College and came to FPCN in 1890. On October 8, 1891, Mrs. Hannah M. Mallmann (probably his wife) was received into the church after being dismissed from the Lafayette Ave. Presbyterian Church in Brooklyn. During that year, a Ruth R. Mallmann also joined FPCN.

Mallmann dedicated the present brown stone sanctuary after preaching the last sermon in the Old White Church in February 1895. Some described Mallmann as a person of enthusiasm and energy; when the walls of a cesspool collapsed one day, he jumped in and helped dig out the workers.

Mallmann left FPCN amid controversy. After he criticized Session members in a sermon, a conference from the presbytery investigated and deemed Mallmann's actions "unwise." Mallmann had to offer a public apology in his "farewell sermon" and was instructed not to occupy the pulpit until that time. After his final sermon, the Presbytery of Nassau declared FPCN's pulpit vacant.

In October 1895, Mallmann left Newtown and became the minister of the Shelter Island Presbyterian Church in New York. His 25-year pastorate there was that church's longest. While there, he became interested in the history of the island, and published a 332-page Shelter Island and Its Presbyterian Church in 1899. In 1910 he organized a United Boy's Brigade, part of a nationwide organization. Twenty teens drilled in uniform and learned "good character."

Mallmann was described as being "gifted as a minister, musician, and a mechanic." He retired to his son-in-law's home in Riverhead, where he died at age 63. He was so attached to Shelter Island that he requested burial in its churchyard.