Daughter of the psychologist Berenice Nunes and André Fronckowiak, Mel Fronckowiak was born in Araranguá, in the state of Santa Catarina, but she grew up in Pelotas, Rio Grande do Sul, the hometown of her family. The actress was quite active in school and from early childhood had always dreamed of having an artistic career. She participated in the Grêmio estudantil and was several times representative of her class. In high school she won a scholarship to a university in Santa Cruz do Sul, where her paternal uncle lived. At 15 years of age, she left home for the first time. She studied journalism in college, with evident support of her family. Her surname, Fronckowiak, is of Polish origin.

Fronckowiak has been in a relationship with Brazilian actor Rodrigo Santoro since 2013. On May 22, 2017, they welcomed their first child, a daughter named Nina.