In the final round, surviving competitor Rosie Brazendale tripped on "profligacy", handing the title to Nicholls.
"I really wanted to win but I never thought I would," said Nicholls.
"Luck played a part as I didn't know some of the words the other competitors got."
Nicholls is the sixth female winner in the past 11 years.
A self-confessed bibliophile, she normally had four or five books on the go and often borrowed the maximum amount of books - 35 at a time - from her library.
"My pocket money usually goes on buying new books and paying library fines."
Spelling Bee Event organiser Janet Lucas said she always found the final intense and this year was no exception.
"Just one letter left off or in the wrong place and you're out. It is really a competition against the dictionary, but the spellers love it. It's fiercely competitive but in a fun way."
Previously, thousands of college students from around the country had competed in a spelling bee test to determine the top 200 spellers, who then advanced to regional spelling bees in August.
The 18 finalists in today's spell-off qualified at these events.