A native of Sewickley Township, Westmoreland County, Pennsylvania, Cowan worked as a carpenter, boatman, and teacher before graduating from Franklin College in New Athens, Ohio in 1839. He studied law with Henry Donnel Foster, was admitted to the bar, and practiced in Greensburg, Pennsylvania. Cowan also became active in politics as a Whig, and campaigned for the party's presidential candidates beginning with William Henry Harrison in 1840. With the demise of the Whigs, Cowan became a Republican in 1855, and supported John C. Frémont for president in the 1856 election.