Theta Xi was founded at Rensselaer Polytechnic Institute (RPI) in Troy, New York, on April 29, 1864, by Peter Henry Fox, Ralph Gooding Packard, Christopher Champlin Waite, George Bradford Brainerd, Samuel Buel Jr., Henry Harrison Farnum, Thomas Cole Raymond, and Nathaniel Henry Starbuck.

Christopher "Kit" Waite, whose father was destined to become Chief Justice of the United States Supreme Court, was from Toledo, Ohio. Peter Fox from Harrisburg, Pennsylvania and Thomas Raymond, a downeastener from Westborough, Massachusetts, together with Waite, were the only Founders from out of state. Nathaniel Starbuck was a local from the campus town of Troy whose father, a leading citizen of the town, was often called upon to address public meetings to arouse popular support for the Lincoln administration. The remaining Founders were all New Yorkers: George Brainerd from Brooklyn; Samuel Buel, Jr. from Poughkeepsie; Ralph Packard from Niagara Falls and Henry Farnum from Port Jervis.