Botswana's Emma Wareus won second place and Venezuela's Adriana Vasini came in third.
China's contestant Tang Xiao was also among the top five.
According to the Miss World website, Mills recently graduated from high school in her hometown of Louisville, Kentucky and has aspirations of becoming a teacher.
For the final, Mills wore a shimmering ivory dress with a slit up the leg.
Mills is taking over the title from 2009 Miss World winner Kaiane Aldorino of Gibraltar.
Women from more than 100 countries participated in the contest, organizers said.
This is the 60th year of the Miss World Competition, and organizers brought back contestants from past decades to give the night a retrospective theme.
The Associated Press contributed to this report.