Kaine is on the Senate Foreign Relations Committee.
The New York Times reported Clinton considered other candidates such as Labor Secretary Thomas E. Perez, Senator Cory Booker of New Jersey, retired four-star Navy admiral James Stavridis who was the supreme allied commander at NATO.
"Mrs. Clinton and Mr. Kaine have similar centrist positions on foreign policy, education and criminal justice and they are said to share an easy rapport and a love of granular policy-making," the Times' Amy Chozick reported.
Clinton said in past television interviews her top concern for a running mate was someone who could step in as president immediately, "if something were to happen to me."
Kaine grew up near Kansas City, where his father owned a metalworking shop. He's pro-life and anti-death penalty, and is a devout Catholic. Kaine, while attending law school at Harvard, took a sabbatical to do missionary work in Honduras. As an attorney he regularly worked on civil rights issues before being elected mayor of Richmond in 1998.