Thomas McKean was the second son of William and Letitia (Finney) McKean. He was of the fourth generation from William McKean of Argyllshire, Scotland, who emigrated to Londonderry, Ireland, about 1674. Coming to Pennsylvania with his mother at an early age (c. 1725), William, the father of Thomas, lived on a farm in Chester County and later became a tavern keeper. His wife, Letitia Finney, belonged to a wealthy and prominent family of Scotch-Irish settlers in Pennsylvania. Thomas was born in New London Township, Chester County.