He was educated in New Athens, Ohio, and later attended Jefferson College, graduating in 1846.
While an Ohio resident, Gilmore married his first wife, Catherine Milligan, in November 1854.
Thomas and his brother Samuel moved to Mahaska County, Iowa, in 1858.
Following the death of his first wife, Gilmore married the Irish-born Ellen Steele in 1860.
[1] After James Addison Young had died in office, Gilmore was elected to the Iowa Senate for District 18 as a Republican for the first time in 1875.