Thomas P. Knox was the son of Andrew Knox Jr. and grandson of revolutionary war hero Andrew Knox. The younger Andrew was a shipping merchant in Savannah, GA when business reverses due tothe War of 1812 drove him into bankruptcy.  He moved to Norristown in 1821 when Thomas was 12 years old and resided with his family at Selma (Gaelic for "High Seat") the former Porter estate until his death. The house being provided for him by his father-in-law as a gift to his daughter Rebecca Rice. Thomas was educated at Norristown Academy and Rutgers's College in New Jersey.

He dedicated himself to the improvement of farming methods. In 1851 Thomas bought out the interest of his mother and brothers and purchased Selma and it's surrounding plantation farmlands. He added to this the Upper Chain Farm buying it from Isaac Williams (purchased originally by John Chain from Mary Norris, widow of Isaac Norris). In 1853 Thomas Knox had civil engineer J. Morton Albertson lay out parts of his farmland in lots for sale leading to the development of the west end of Norristown. Samuel E. Hartranft, father of eventual general and governor John Hartranft bought a portion as well as 15 lots going to B. Markley Boyer, later Montgomery County president judge. A lifelong Democrat Thomas Knox was appointed the Penna. governor's aide and was elected to the state senate serving 3 years.

In 1855 he was chosen president of the state agricultural society. He also served as Norriton Justice of the Peace before the boundaries of Norristown were extended to include his home. 3 of his children died in childhood. This and the death of his wife Sarah Ann Leedom devastated Knox. A surviving daughter named Ellen married Joseph Fornance. Thomas Knox was described as a man of medium stature, fair complexion, suave, courtly, an agreeable companion who in later years sported snow white hair. Knox died May 27, 1879 at age 70 and is buried in Montgomery Cemetery.