Mann v. Rock Island Bank was a United States Supreme Court case that was decided in 1870. The case involved a dispute between the plaintiff, William Mann, and the defendant, Rock Island Bank. Mann had deposited money with the bank and had received a certificate of deposit. The bank then failed and Mann was unable to recover his money. Mann sued the bank for the return of his money, but the bank argued that it was not liable for the money because the certificate of deposit was not a negotiable instrument. The Supreme Court disagreed and held that the certificate of deposit was a negotiable instrument and that the bank was liable for the money. The Court held that the bank was liable for the money because it had accepted the certificate of deposit and had not given Mann any notice that it was not a negotiable instrument. The Court's decision in Mann v. Rock Island Bank established that a certificate of deposit is a negotiable instrument and that a bank is liable for the money if it accepts the certificate of deposit without giving the depositor any notice that it is not a negotiable instrument. This decision has been cited in numerous cases since then and has been used to establish the rights of depositors in similar cases.

In Mann v. Rock Island Bank, the Supreme Court was asked to decide whether a bank could be held liable for failing to pay out on an insurance policy that had been assigned by its original owner and endorsed in blank. The majority opinion found that the bank was not liable because it had no knowledge of the assignment and endorsement when it received payment from the insured party's estate after his death. However, Justice Field dissented from this decision, arguing that banks should have a duty to inquire into any assignments or endorsements made with respect to policies they receive payments for before disbursing funds. He reasoned that since banks are expected to exercise reasonable care in their dealings with customers' money, they should also take steps to ensure proper handling of insurance policies as well. Furthermore, he argued that if banks were allowed such wide discretion without consequence then there would be little incentive for them act responsibly when dealing with these matters in future cases.