Robert was the son of John M. Calhoun and Susan Tracy. He married Mary Susan O'Brien on Jan 3, 1854. They had 4 children. Mary Susan died by 1864 when he married her cousin, Adeline O'Brien with whom he had 6 more children. He served with two of his brothers in the Union Army during the Civil War. Robert was a Corporal in Co. E, 39th Missouri Infantry.