In 1841, Thomas was elected Governor of Maryland, defeating challenger William Cost Johnson by a margin of 600 votes.

He was also a staunch opponent of slavery, a unique position in a border-state like Maryland, decrying it as "altogether unworthy of enlightened statesmen, and should be by all patriots repudiated".

He served as governor from 1842 until 1845, narrowly beating William Cost Johnson,[3] who he succeeded as Maryland's 6th district congressman, in 1841 for a three-year term.

Thomas was appointed by President Grant to serve as the United States Minister to Peru, and help this position from March 25, 1872 to July 9, 1875.

On January 22, 1876, while overseeing improvements on his estate near Frankville, Maryland, a community that once existed along the Baltimore and Ohio Railroad in Garrett County, Thomas was killed instantly when he was struck by a locomotive.