Mackay will join this summer as marketing director, working with the sales team to provide support to financial advisers.
A J Bell manages more than 29,000 individual Sipps and has over £5bn in pension assets under trusteeship.
Mackay has been working at Skandia for the past 11 years apart from a brief departure to Barclays Wealth Management in September 2006, where he was head of retirement for a month.
He was elevated to the role of head of marketing across life and pensions when he returned to Skandia, having been pension marketing manager prior to his departure.
AJ Bell actuary and chief executive Andy Bell says: “Given the strong growth of the business over the past couple of years, we felt that it was important to hire a specialist to the position of marketing director, as I would say that our strong growth has been despite rather than because of our marketing capabilities.
“In Billy, we now feel that we have the right person with the knowledge to drive our offering forward.”