Herglotz conservation laws of nonholonomic mechanical systems in event space are studied. The Herglotz generalized variational principle in event space is given, and the Herglotz -d' Alembert principle, a new differential variational principle for nonholonomic mechanical systems in event space, is derived by introducing non-holonomic constraints and using the Holder definition of commutative relation. The transformation of the invariance condition of Herglotz-d' Alembert principle is established by introducing space generators and parameter generators in the event space. Herglotz conservation theorem and its inverse for nonholonomic nonconservative mechanical systems in event space are constructed based on this principle. As particular cases, the Herglotz conservation laws in configuration space and the Herglotz conservation laws for holonomic mechanical system in event space are given. An example is given at the end of the paper to illustrate the application of Herglotz conservation laws.