Mimecast has appointed Leslie Nielsen as its new Chief Information Security Officer (CISO). The company announced the move on August 28, 2025, reinforcing its focus on human risk and AI-powered defenses.
Nielsen brings more than 25 years of cybersecurity experience. He most recently served as SVP and CISO at Klaviyo, where he strengthened security operations and prepared the company for IPO readiness. Under his leadership, Klaviyo scaled to $1.3 billion in revenue.
As CISO of Mimecast, Nielsen will oversee global cybersecurity strategy. His role covers threat intelligence, risk assessment, incident response, and regulatory compliance. He will also lead resilience initiatives to safeguard over 42,000 customers worldwide.
Mimecast CEO Marc van Zadelhoff expressed confidence in Nielsen’s appointment. He said Nielsen’s proven leadership in scaling security functions makes him an ideal fit for the company’s mission.
Nielsen shared his enthusiasm for the role. He said defense strategies must put people at the center, and Mimecast’s human-centric approach positions it well in the age of AI-powered security threats.
Before Klaviyo, Nielsen held senior leadership roles at Nuance Communications, SAP Concur, SuccessFactors, and SOS Security. He also worked at IBM and ACS, and was part of Google’s CISO Cloud Advisory Board.
