 McMullen became a member of the Senate of Virginia in 1839, serving until 1849.He was elected as a Democrat to the United States House of Representatives in 1848, serving from 1849 to 1857.There, McMullen served as chairman of the Committee on Expenditures in the Department of the Navy from 1851 to 1855 and chairman of the Committee on Expenditures on Public Buildings from 1855 to 1857.