Mark Spencer Brewer (October 22, 1837 – March 18, 1901) was an American lawyer and politician from the U.S. state of Michigan who served four terms over two different stints in Congress between 1877 and 1891.

Brewer was born in Addison Township, Michigan, and attended the rural schools and Romeo and Oxford Academies.

He was city attorney of Pontiac in 1866 and 1867 and circuit court commissioner for Oakland County 1866-1869.

He declined to be a candidate for renomination in 1890 and resumed the practice of law in Pontiac.

He was a delegate to the 1896 Republican National Convention and was appointed a member of the United States Civil Service Commission by President William McKinley January 18, 1898, and served until his death in Washington, D.C.