The so-called principal recruitment partner deal will see Core-Asset seek out candidates for positions across a range of disciplines and on a mixture of retained fee and "no win no fee" arrangements, where it gets paid if a candidate gets a job.
Additional services will include salary benchmarking, market insights, candidate testing, succession planning and talent management advice.
Subsidiaries including Alliance Trust Investments and Alliance Trust Savings are included in the arrangement.
Betsy Williamson, managing director of Core-Asset Consulting, said: "Alliance Trust's overall objective as an organisation is to increase its assets under management to £20 billion by 2020.
"That is a significant growth trajectory and will require substantial additional recruitment."
This is the first such arrangement that Core-Asset has undertaken.
The business claims it is unique in the Scottish financial sector.
Typically companies hire different agencies to hire staff in specific disciplines.
Ms Williamson said: "This requires a huge amount of technical knowledge of the industry.
"It requires the consultant to work incredibly closely with that business and to understand the requirements of that business. It is incredibly difficult to do."
Ms Williamson has worked in the sector since 1999. Core-Asset was established in 2005 and has grown from three members of staff to 22.
They operate in specialist teams covering areas including asset management, life and pensions and accounting and finance.
Ms Williamson said this range of specialisms was vital to being able to offer the full recruitment service to Alliance Trust.
She drew an analogy with the large accountancy firms who have expanded into a variety of business services required by their clients.
Ms Williamson said:"Our consultants operate on a multi-discipline, sector-specialist basis, contrary to the single-discipline, multi-sector approach which characterises much of the recruitment industry.
"This means we can support Alliance Trust on an exclusive basis, and in doing so offer high value and high engagement. This is more in line with how other professional service firms support their clients.
"As its principal recruitment partner we will have a much closer relationship with Alliance Trust, allowing us to develop a more detailed understanding of their business objectives and helping us to better anticipate future resourcing needs."
Ms Williamson believes similar arrangements could work elsewhere in the Scottish financial services industry.
"Ultimately, I believe our partnership will lead to increased recruitment efficiency, reduced bottom-line costs and a further enhancement of Alliance Trust as a leading employer in Scotland and beyond," she said.
Lynne Dalgarno, human resources director for Alliance Trust, said: "Their unique model will allow us to bring greater efficiency to our recruitment activity and ensure that we have access to the best possible candidates for the wide variety of roles that we recruit for."