Austin was the son of Betty, a dower slave who brought him as a baby when she came to Mount Vernon upon the marriage of Martha Custis and George Washington in January 1759. The identity of his father is not known, but Austin was of mixed racial heritage. Beginning in the mid-1770s, he worked as a waiter in the mansion at Mount Vernon, and appears to have also worked as a postillion or footman for the carriage. During the Revolutionary War, Austin may have been the young man who accompanied Martha Washington on at least one of her visits to her husband in the field. He was trusted to make long rides on his own, including trips from Philadelphia to Mount Vernon.
Austin was the half-brother of Oney Judge. He married another dower slave at Mount Vernon, possibly a seamstress named Charlotte. (Moll, whom some historians have assumed had been his wife, was about 18 years his senior.) Austin seems to have been a stableworker at the presidential residences in New York City and Philadelphia. In the President's House, he probably slept in the slave quarters adjoining the stables, with Giles and Paris.
Austin died on December 20, 1794, after a fall from a horse near Harford, Maryland. He was survived by his widow and five children: two sons, Billy (born ca. 1782), Timothy (born 1785), and three daughters, Elvey, Jenny and Eliza (born between 1786 and 1795). Austin's children seem to have been inherited by G. W. Parke Custis after Martha Washington's death in 1802, and probably were moved to Arlington House (now Arlington National Cemetery). It is not known what became of his widow.