Assurance cases are often required as a means to certify a critical system. Use of formal methods in assurance can improve automation, and overcome problems with ambiguity, faulty reasoning, and inadequate evidentiary support. However, assurance cases can rarely be fully formalised, as the use of formal methods is contingent on models validated by informal processes. Consequently, we need assurance techniques that support both formal and informal artifacts, with explicated inferential links and assumptions that can be checked by evaluation. Our contribution is a mechanical framework for developing assurance cases with integrated formal methods based in the Isabelle system. We demonstrate an embedding of the Structured Assurance Case Meta-model (SACM) using Isabelle/DOF, and show how this can be linked to formal analysis techniques originating from our verification framework, Isabelle/UTP. We validate our approach by mechanising a fragment of the Tokeneer security case, with evidence supplied by formal verification.