Chairman Ron Baxter will become the acting chief executive as an interim measure and assume the role in addition to his chairman duties. Mr Mill was with the company for over seven years in total.
Mr Baxter was a former senior adviser at the Financial Conduct Authority (FCA), the Prudential Authority, Bank of England, and a former chief executive and non-executive director of financial services businesses.
Mr Baxter, chairman of Alliance Trust Savings said: “On behalf of the board, I would like to extend our thanks to Patrick for his contribution over the last seven years, during which time assets under administration have increased to over £15 billion.
“We wish him well in his future endeavours. We have a strong management team and excellent staff to take the business forward.”
Mr Mill assumed his role as chief executive Alliance Savings Trust in 2012, following his role as sales and distribution director for two and half years.
The company released the statement this afternoon, however no quote from Mr Mill was included.
Alison Fowler, the company’s chief operating officer, left the company in August following eight months in her role.