Jo Hopper hired the bank to administer the US$19mln estate of her late husband Max back in 2010 after his sudden death from a stroke.
However, instead of doing its job, a Dallas jury found that JP Morgan committed fraud, breached its fiduciary duty and broke a fee agreement.
The court heard how it took years for assets to be released, so long that in some cases stock options expired.
The bank also ignored requests from Mrs Hopper to sell certain shares and used the estate’s proceeds to pay its legal fees.
“The bank took years to release basic interests in art, home furnishings, jewellery and notably Mr Hooper’s collection of 6,700 golf putters and 900 bottles of wine,” read a statement from Hopper’s attorneys.
“Even today some assets – now more than seven years after Mr Hopper’s death – still have not been released to Mrs Hooper.”
JP Morgan has said it believes the verdict will be overturned.
Shares were up 0.1% to US$95.27 in pre-market trade on Thursday.