A spokesperson for Janus Henderson confirmed that Hillenbrand will be leaving the company "to pursue opportunities outside of the firm", effective 31 December.
Hillenbrand has served as head of retail since joining the firm in 2009, prior to which he held roles at New Star Asset Management and Invesco Perpetual.
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The spokesperson for Janus Henderson said the company would begin a search for his replacement "shortly", initiate the recruitment process for both internal and external candidate via a recruitment firm.
They confirmed that in the interim period Hillenbrand would continue in his current role and would be working "closely with his team, supporting them and transitioning his responsibilities until the end of the year".
"The UK intermediary team remains a key strategic priority for Janus Henderson, and we will continue to invest in the business and top talent to help support the needs of our clients and their clients," they said.
"We thank Simon for his considerable contribution to the business over many years and his unwavering commitment to our clients, and we wish him well for the future."