Investigators found that his wife Brenda died in January 2010 but kept observations as he went to the Post Office to claim her benefits.
Mills admitted 14 fraud charges and will be sentenced at a later date.
Flintshire magistrates heard that the couple had been married 44 years but that Mrs Mills had been extremely ill for the last five years of her life, ending her days in hospital.
Prosecutor Henry Hills said that the Department of Work and Pensions received anonymous information that he was continuing to claim his dead wife's benefits.
Checks showed that retirement benefits of £40 a week were still being paid out in her name together with £402 a month disability living allowance - but checks with the registrar showed that she died on 23 January 2010.
Observations showed that he was attending the Greenfield sub post office weekly, and post office records showed he was still claiming benefits as a married man caring for a disabled wife, as well as her benefits.
After Mills was arrested in January this year, it also emerged that he had failed to disclose his miner's pension.