Mr Kaine’s upbeat remarks contrasted with Republican Donald Trump’s pessimistic convention speech.
The 58-year-old moderate Democrat could appeal to Republicans who have been unsettled by Mr Trump’s candidacy.
However, the pick is seen as a blow to more left-leaning Democrats.
Mrs Clinton’s one-time rival, Senator Bernie Sanders, waged a strong campaign, championing liberal causes such as universal health care coverage.
In part, Mr Kaine’s introduction sought to assure those voters of the senator’s left-wing credentials.
“If you want to be right, be a pessimist, if you want to do right, be an optimist,” Mr Kaine said, recalling his time fighting housing discrimination as a young lawyer. “I like to do right.”
Mrs Clinton said Mr Kaine was “everything Donald Trump is not”.
“He is a progressive who likes to get things done. That’s just my kind of guy,” Mrs Clinton said.
The Trump campaign attacked the choice, dubbing the senator “Corrupt Kaine”.
As governor of Virginia, Mr Kaine accepted about $160,000 worth of gifts from political supporters, which was legal under the state’s lax gift laws.
“If you think Crooked Hillary and Corrupt Kaine are going to change anything in Washington, it’s just the opposite,” said Jason Miller, spokesman for the Trump campaign.
On Twitter, Mr Trump sought to drive a wedge between the Clinton campaign and voters who supported Mr Sanders in the Democratic primary.