Previously assistant manager on both funds, Cassell has worked with investment director and head of UK equities Mark Martin since 2014.
She joined Neptune in 2013 and qualified as a chartered accountant at Deloitte.
Neptune founder and CEO Robin Geffen said: “Holly’s contribution to the Neptune UK Mid Cap and UK Opportunities funds has been vital, and this promotion is recognition of that.
“She is recognised as one of the UK’s best up and coming fund managers and has a deep understanding of the UK market.
“At Neptune we pride ourselves in nurturing home-grown talent, and we have every confidence that Holly will excel in her new role.”
Yesterday, it was confirmed that Neptune China fund manager Douglas Turnbull had left the firm, with the portfolio being taken over by head of emerging markets Ewan Thompson.
