Thomas, who is Karamu's first woman principal, said she plans to demonstrate to students what it means to be "Proudly Karamu" by launching the Tira Ora Karamu Alumni Association.
"By connecting our students with their forebears, I see this as a positive way to engage, motivate and ignite their potential by supporting and enticing students into the different industry, trade, business and professions through education and career pathways," she said.
"I want our school vision to truly come alive and who better to lead that than someone who has come from here and that's something I can do."
The new principal, who was the school's deputy head girl, started her first teaching position at Karamu in 1988, as well as teaching at Woodford House for 16 years, including six as deputy principal.
Karamu Board of Trustees chairman Dave de Lange said Thomas brings a wealth of educational knowledge and leadership to Karamu.
"Dionne has a great affinity with Karamu High School and our community and we're looking forward to working together in her new role," he said.
"She's committed to educational excellence, supporting students to achieve to their full potential while driving the values and culture that makes Karamu a great school."