Listed by Forbes this year as the world's 195th richest billionaire with a fortune of $6.1 billion (€4.6 billion), the highest-placed among Spanish women, she was also described as the richest self-made woman in the world.
Mera dropped out of school at 11 and started work as a seamstress at 13 before going on to found the Inditex textile giant alongside her husband Amancio Ortega, whom she later divorced. Ortega is listed by Forbes as the world's third-richest man with $57 billion.
Inditex released a statement expressing the group's sadness over Mera's death.
"The group wishes to send its sincere condolences to her loved ones and friends at this extremely difficult time, after the loss of a person who contributed so much to the origins and development of the company," it said.
Besides Zara, the Inditex group owns a number of successful international brands including Massimo Dutti and Pull and Bear.
Mera had left the Inditex board in 2004 but kept a 5.05% stake in the firm, according to conservative Spanish daily ABC. She also held 30.6% of hotel chain Room Mate, it said.