Rowling's phenomenally successful series of books, which are estimated to have earned her £500m, have helped the author overtake Cherie Blair, second on the UK list, who fell 50 places this year to 62nd in the world. Rose Marie Bravo, the chief executive of Burberry, is third in Britain, 63rd overall while the Queen is the fourth Briton and a humble 75th, globally. These four are the only Britons to make the world top 100. Irish President Mary McAleese romped home ahead of them all, ranked 21st.
Top of the pile for a second year running is US Secretary of State Condoleezza Rice, who became the first African-American to take the post in January this year. She is closely followed by Wu Yi, China's Vice Premier and Minister of Health, known as the country's Iron Lady. Yulia Tymoshenko, the Ukrainian Prime Minister occupies third place.
The original Iron Lady, Margaret Thatcher, has dropped off the list, having come in at 21st last year, one place above the Queen.
Elizabeth MacDonald, co-author of the list, confirmed that JK Rowling's success stemmed from the wealth she had amassed through worldwide sales of Harry Potter books: "Cherie Blair does not have that much power anymore," she said. "We delved deep into their resumés to look at how much clout these women carry in the market place."
Ms MacDonald said the Queen had slipped down the list because she was no longer as active as she used to be while Mary McAleese's success was borne out of Ireland's economic boom.
"She is the first native of Northern Ireland to hold the position and has been hailed the People's President," she said. "There are a lot of newcomers with very powerful resumés. We are looking for women who make it to the top culturally and economically."
Ms MacDonald, a senior editor at Forbes, said the list spanned the global and cultural spectrum, encompassing humanitarians, politicians and businesswomen.
She hailed the success of Ms Tymoshenko, who was in third place after helping to lead the Orange Revolution in a deeply divided Ukraine. "She used her position to forcibly shake up the world around her," Ms MacDonald said. "That is what we are looking for in our powerful women - not just those who are aspirational but those who are up there doing the work, those who are inspirational and can act as role models."
To compile the rankings, the magazine looked at public visibility, measured by press exposure, and economic impact.
The remainder of the Forbes top 10 were: Gloria Arroyo, President of the Philippines, at number four, Margaret Whitman, chief executive of eBay, at five, Anne Mulcahy, chief executive of Xerox, Sallie Krawcheck, chief financial officer of Citigroup, Brenda Barnes, chief executive of Sara Lee, Oprah Winfrey, US chat show host and chairman of Harpo, and Melinda Gates, co-founder of the Bill and Melinda Gates Foundation.