In 2011, the mummified body of a Black adult female was discovered in a mid‐19th‐century Fisk iron coffin buried in Queens, New York City. Archival research points to the identity of the woman as Martha Peterson, a domestic worker who lived in the home of William Raymond, the business partner and neighbour of Almond Fisk, the coffin inventor and manufacturer. Here, we apply biogeochemical methods to provide information about the Woman in the Iron Coffin's (WIC's) residence, diet, and health to complement the biological profile and test the archival‐based interpretation of identity. We conducted stable isotopic (δ13C, δ15N, δ18O, 87Sr/86Sr, and 206Pb/207Pb) and elemental (As, Sr, and Pb) concentration analyses of a second premolar and one strand of hair to characterize her lifeways during middle childhood and near the time of death. We interpret WIC's geographic location during the time of tooth formation as local to the New York region, compared with established δ18O and 87Sr/86Sr isoscapes of the United States and 206Pb/207Pb values of regional coal and ore. WIC's δ13C and δ15N values indicate a mixed diet similar to contemporaneous peoples from other Mid‐Atlantic States. Pb concentrations are potentially indicative of acute Pb‐caused health problems in a period of ~1 year during middle childhood. Our biogeochemical results bolster archival‐based interpretations of WIC's identity as a member of the Peterson's, a local free Black family, and provide a glimpse into the life of a Black woman in 19th‐century New York City.