Miller, 28, announced the passing of 29-year-old Donna, with whom he had two children, on his Twitter account on Wednesday.
He wrote: "Saddest day of my life.RIP Donna.......u will always b loved!!..a wonderful mum 2 our beautiful boys!!..they will never 4get u!!..xxx"
The cause of death has not been made public but the Cumbrians did release a statement.
It read: "The thoughts of the directors, staff and everyone connected with Carlisle United Football Club are with Lee Miller and his family at the sad news of the passing of his partner Donna last night (Tuesday).
"The club is sure that everyone will respect the privacy of the family at this very sad time."