Mr Mill previously worked at the firm as distribution director and has over 20 years financial services industry experience.
His previous roles have included sales director at Prudential and managing director of financial services at Bradford and Bingley.
Previously, Rob Burgess was chief executive of Alliance Trust Savings but the company has said this role will not be replaced.

Mr Mill's appointment comes at the time when the firm is looking to move towards profitability and expand into the advised market.
Katherine Garrett-Cox, chief executive of Alliance Trust, said: "Patrick has been instrumental in the growth of our savings business over the last two years and has proven in his role as distribution director that he has both the experience and leadership skills to drive the business forward and deliver long-term value for our shareholders."
Mr Mill said: "The RDR and the growth of the platform market present considerable opportunities to grow our business further as advisers seek to maximise value for their clients and continue to move assets onto investment platforms.
"We also expect an increase in our direct customer base as RDR starts to highlight the true cost of investing and makes clear the value of Alliance Trust Savings."