During the American Revolutionary War Clay joined the Ninth Virginia Regiment on October 1, 1776, as an ensign and probably fought in the New Jersey and Pennsylvania campaigns (but was on detached duty so not present at the disastrous Battle of Germantown).

[1] On December 4, 1788, Mathew Clay married Mary Williams of Pittsylvania County, who died on March 25, 1798, after giving birth to two sons and two daughters.

[1] As indicated by his will discussed below, Clay may also have had children out of wedlock, and directed his sons who were his executors to provide for three girls and one boy when they reached legal age for their gender.

[4] In 1790, in addition to service on the Committee of Propositions and Grievances, Clay became a member of a commission to improve the upper Roanoke River to permit commercial navigation.

The next year, Clay headed a commission to determine whether to remove dams on the Bannister River to permit upstream fish migration (the harvest of shad, a large herring being important both as subsistence and for sale in the era).

Clay also supported expanding suffrage to all white men who either owned property or could be forced to bear arms for the community, basing his argument on fairness.

Clay had a combative debating style, and in one legislative argument in the 1791 session, accused Pittsylvania's other delegate of improprieties (but later apologized formally to the House for his conduct).

[7] Clay won back his seat in the April 1815 (but was never sworn into the Fourteenth Congress), but died suddenly at Halifax Court House, while returning home from Richmond.

His will also directed his two sons (as his executors) to give a fourteen-year-old boy named William Penn a horse, bridle, saddle, clothes and $1000 when he reached 21 years old.