Harrison v. Myer, Executrix was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to modify a will. The case involved the estate of a deceased man, John Harrison, who had left a will that provided for his widow, Mary Harrison, to receive a life estate in his property. Mary's son, John Myer, was named as executor of the will. After Mary's death, John Myer sought to modify the will in order to give himself a larger share of the estate. The Supreme Court held that the state court did not have the authority to modify the will, as it was a matter of federal law. The Court held that the will was a contract between the testator and the beneficiaries, and that the state court could not interfere with the terms of the contract. The Court also held that the executor had no authority to modify the will, as it was the testator's intent that the will remain unchanged. The Court concluded that the state court had no authority to modify the will, and that the executor had no authority to do so either.

In the case of Harrison v. Myer, Executrix, the Supreme Court was tasked with determining whether a contract between two parties should be enforced despite one party’s failure to comply with certain conditions outlined in the agreement. The majority opinion held that since there had been no substantial performance on either side, neither party could enforce their rights under the contract and it must be considered void. However, Justice Field dissented from this decision and argued that although both sides had failed to perform some of their obligations under the contract, they were still bound by its terms as long as each fulfilled at least part of what they agreed upon. He further noted that if only one side performed any part of their duties then they would have a right to seek enforcement against any breach committed by other parties involved in order for them to receive compensation for damages suffered due to such breach. Ultimately he concluded that even though both sides did not fully comply with all aspects outlined in the agreement, it should still be enforced so long as each party completed some portion of what was required from them according to its terms.