At age twenty-two, Thomas began working as a druggist, opening his own store in Philadelphia, and six years later, in 1820, he married Mary Ann Wilson in Burlington, New Jersey.

[2] In 1827, M'Clintock co-founded the Free Produce Society of Pennsylvania along with James Mott, Richard Allen, and others and became its first secretary.

This predominantly Quaker movement, which included also free African Americans, was an effort to promote the exchange of goods not involving any slave labor.

This separation among the Quakers resulted from disagreements as to what role doctrine should play in the church, how much one should participate in social activism, and other factors.

In 1843, Thomas was elected to the board of managers of the American Anti-Slavery Society and later served as a vice president for a number of years.

Thomas and his wife organized anti-slavery petitions, gave refuge to black children, and continued the Free Produce movement.

[6] The M'Clintocks played important roles in the 1848 schism of Hicksite Quakers in western New York and the subsequent formation of the Yearly Meeting of Congregational Friends.

The home in Waterloo, New York where the family resided during July 1848 was listed on the National Register of Historic Places in 1980 as the M'Clintock House.