Thomas Leech was probably born in Philadelphia, the son of Toby and Hester Leech, who had come to Pennsylvania with William Penn. Leech settled in Philadelphia county’s Cheltenham township, but was a vestryman and warden of Christ Church in the city of Philadelphia. He spent thirty years representing Philadelphia county in the provincial assembly.
Leech was a founder and trustee of the College and Academy of Philadelphia (now the University of Pennsylvania) until his death in 1762. In 1756 Thomas Leech paid the Academy tuition of student Thomas Leech.