Career profile
Leslie Nielsen
Chief Information Security Officer (CISO), Mimecast
- Current role, per sources checked 16 October 2025
- 27 August 2025
Leslie Nielsen is the Chief Information Security Officer of Mimecast, appointed in August 2025. In the role, he leads the company's global cybersecurity strategy, including threat intelligence, risk management, and detection and response initiatives, and he oversees regulatory compliance for Mimecast's customer base of more than 42,000 organizations.
Career path
Nielsen began his senior security career with roles at IBM and at ACS, a company later acquired by Xerox. He went on to serve as Chief Technology Officer at SOS Security. He then held SVP and CISO roles at SuccessFactors and SAP Concur. He subsequently served as SVP and CISO at Nuance Communications, where he led the company's cybersecurity practice through a transformation that contributed to Nuance's acquisition by Microsoft. From Nuance he moved to Klaviyo, a marketing automation platform, as SVP and CISO, a period in which he helped scale the company to $1.3 billion in revenue and led its security function through IPO readiness. He joined Mimecast from Klaviyo in August 2025.
Nielsen holds a Bachelor of Science in Computer Science from Texas Tech University. He has also served as a member of Google's CISO Cloud Advisory Board and has sat on enterprise architecture and cybersecurity steering committees for Fortune 500 companies.
Public statements and published work
On joining Mimecast, Nielsen said, "I'm thrilled to join Mimecast on their mission to secure the human layer in the new age of AI-powered security threats," adding that defense strategies must place humans at the center of security.
Nielsen has been invited to speak at multiple conferences on security, privacy and policy topics. He also contributes to Mimecast's company blog; a January 2026 post under his byline, Global games mean cybercriminals can also go for gold, addressed cybercrime tied to global sporting events.
