World number one Williams scored her second Roland Garros win, beating defending champion Maria Sharapova of Russia 6-4 6-4.
The 31-year-old American, who won her first Roland Garros title in 2002, became the oldest player since tennis turned professional in 1968 to lift the Suzanne Lenglen Cup after overpowering the second seed on Court Philip pe Chatrier.
Williams, who now has 16 grand slam titles to her name, recovered from a slow start to take the opening set after breaking three times.
She ended the contest on her first match point with an ace after one hour 46 minutes.
Fans watching the match on a giant screen erected in front of Paris’s town hall said the fact that it was a head-to-head with tennis’s top talent made for a great game.
Others were underwhelmed, saying the game played by pre-match favourite Williams was predictable.
But happily, Norwegian fan Charlotte Ringdal who had come hundreds of miles to soak up the atmosphere said it was money well spent.