He worked on his father's farm until he was of age, and then studied and taught until he entered the Lawrence Scientific School of Harvard University, where he graduated in 1851.

His ability as a mathematician led in 1849 to his appointment as assistant in the preparation of the American Ephemeris and Nautical Almanac, in which he continued to engage until 1884.

In the town of Brookline, Massachusetts, Runkle was a chairman of the School Committee and an early advocate of mathematics and technical education.

This rendered him familiar with the legal questions involved in tariff and internal revenue litigation, and resulted in his devoting himself largely to that class of business.

Cornelius's wife, Lucia Isabella Gilbert Runkle (born in North Brookfield, Massachusetts on August 20, 1844), was an editorial writer and contributor to the Tribune.