Franklin only swam 13 minutes earlier in the 200m freestyle semi-finals but still had enough energy reserves to finish the better and down Australia's Seebohm Emily in 58.33sec.
Seebohm was second in 58.68sec with Japan's Aya Terakawa getting the bronze.
The 17-year-old from Colorado could swim four individual events and three relays in London, a programme that would make her the first US woman to swim seven events at one Olympics.
"I just did everything I could to get my hand first to the wall," rising star Franklin said. "I love doing back-to-back doubles, it's fun and it doesn't allow you much time to get nervous."
Showing off her gold medal to reporters, voluble Franklin gushed: "It's right here, isn't it pretty? Finally got one after 17 years!"
And the bubbly teenager added: "I saw my parents' reaction on the screen and I just started bawling.
"I couldn't be happier. I've just won an Olympic gold medal."
Franklin envisioned before the final that she would have to hunt down the front-running Seebohm.
"I knew Emily Seebohm had taken out that heat, so I knew I needed to be out there and give it everything I had coming home," she said.
Franklin comes from Aurora, outside Denver, where 12 people were shot dead and 58 left wounded by a gunman at a cinema earlier this month.
The teenager said last week she planned to dedicate her wins to Colorado following the tragedy.
"Hopefully, I can make my state proud and give a little bit of fun for this really tough summer that Colorado has had," she said.
Meanwhile beaten rival Seebohm had swum an Olympic record in the semi-finals of 58.23sec but could not withstand Franklin's powerful finish.
"I am disappointed, but at least I have finished with a silver," Seebohm said. "But it is huge motivation to keep swimming and to make me work harder over the next four years.
"Last year was a very tough one for me, I had swine flu which had me in isolation for four days in hospital.
"Then I had six bouts of tonsilitis, then I went to USA on a training camp and I got pancreatitis, which meant I vomited every time I ate.
"But no excuses. I went in there and raced, but I didn't come out on top."
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