Molly was born in 2368, to Miles and Keiko O'Brien. She was delivered by Worf in the U.S.S. Enterprise-D's Ten-Forward lounge and later moved to Deep Space Nine with her parents.

In 2374, Molly went on a picnic to the planet Golana, with her parents and younger brother, Kirayoshi. During this excursion, she fell through an abandoned time portal and ended up 300 years in the past. Her parents managed to rescue her, but their calculations were off, and Molly returned to them as an eighteen year old.

This Molly, who was without human contact for a decade, was unable to re-adapt to society and life on DS9. The O'Briens eventually sent their daughter back to Golana's past, where she spotted eight-year-old Molly moments after her fall through the portal. Recognizing her younger self, Molly managed to send eight-year-old Molly back to her parents.

One of Molly's favorite dolls on DS9 was named Lupi.