[2] On the termination of the survey in 1841, he entered Princeton Theological Seminary while also assisting Professor Rogers in preparing the final report and map of Pennsylvania.

He then worked for two years for the American Tract Society, and at the close of 1847 he joined Professor Rogers again in preparing geological maps and sections at Boston.

He spent 1863 in Europe, examining the Bessemer ironworks in Sheffield for the Pennsylvania Railroad, and in 1867 he was one of ten commissioners sent by the United States Senate to the World's Fair in Paris.

Lesley was a proponent of the high antiquity of humankind; "My own belief is but the reflection of the growing sentiment of the whole geological world [...] that our race has been upon the earth for hundreds of thousands of years.

The couple's daughter was the painter Margaret Lesley Bush-Brown,[5] whose first job was creating geological models for her father.