Mark Kelso was born January 12, 1803, to John N. Kelso of Cumberland, Pennsylvania, and Mary McCormick Kelso of Lancaster, Pennsylvania. John and Mary must have moved to Allegheny County before 1788, as their first child, George, was born that year in that county.
Mark Kelso is reported working as a farmer in Upper St. Clair Township in the 1850 census. His large farm was located near the present day Kelso Road in Mt. Lebanon close to the Scott Township border. Mark married Mary Borland of Carlisle, Pennsylvania, on September 1, 1836. They had six children-all born in Allegheny County. Their daughter Mary Ann, who married farmer John N. Woods on March 29, 1859, died New Year’s Eve 1862, at the age of 22. Mark and Mary’s son Joseph was living near New Galilee, Pennsylvania, when he died February 18, 1852, at the age of 46.