Mathias' brother, Isaac McGirk, represented Marguerite Scypion in her claim for freedom in the Missouri courts in 1805.
An 1892 biography provided this sketch: When Governor McNair selected the first Chief-Justice of the Supreme Court of Missouri, he set the worthy example of eschewing political considerations.
Judge McGirk was born in Tennessee in 1790, came to St. Louis about 1814, but soon removed to Montgomery County in the interior of the State.
He was appointed to his high position at the early age of thirty-one, and filled it admirably and well for twenty years.
Prior to his appointment, he had served in the Territorial Legislature, and was the author of a number of the important statutes enacted by it, notably the one introducing the common law of England into the local jurisprudence.
In differing from his associates upon the nature of the legislation under which loan-office certificates were issued, holding that, in the light of the Constitution of the United States, they were bills of credit, he expressed the views which received the sanction of the Federal Supreme Court in the well-known case of Craig v. Missouri, 4 Peters, 410.
At his handsome home on Loutre Island in the Missouri River, he was hospitable to his friends, and drank deep of the sober pleasures of a full, rounded life.
There was no brilliancy about him, but he had good, practical sense, with a naturally strong and vigorous intellect, and a fine, retentive memory, which enabled him to gather a large amount of legal learning.
In politics he was an Old-Line Whig, very decided in his views, and probably would have been very active but for the restraint imposed by his official position.