 Calvert was elected as a Unionist to the Thirty-seventh Congress representing Maryland's Sixth Congressional District in an election held on June 13, 1861.In the election of November 4, 1863, he sought reelection as a Conditional (Conservative) Unionist candidate in the Fifth Congressional District (the Sixth District having been abolished), but was defeated.After leaving office, he resumed agricultural pursuits until his death on May 12, 1864 at Riversdale, and is interred in Calvert Cemetery.