The former director of technical services has agreed to join an undisclosed wealth management firm after his employment with Dentons ends in January.
He is currently on gardening leave.
Mr Tilley will be succeeded by Stephen McPhillips who has been appointed as technical sales director at the Sipp and Ssas firm.
Mr McPhillips has been with Dentons since January 2012, having spent all of his working life in the member directed pensions sector including at Pointon York, Cofunds, James Hay and Scottish Equitable.
He is a trained Ssas administrator and was elected a Fellow of the PMI in March 2018.
Derrick Fowler, joint managing director at Dentons Pensions Management, said: "Having spent 30 years with us, Martin has decided to move into a wealth management role and will be leaving Dentons in January.
"He goes with our very best wishes and we thank him for his contribution to the business.
"With immediate effect, Stephen McPhillips has been appointed Technical Sales Director; with over 3 decades of Sipp and Ssas experience he was an obvious choice to move into this position. His technical knowledge is second to none and we look forward to continuing to provide expert help and support to advisers and their clients."
Mr McPhillips added: "There has been unprecedented change in the Sipp and Ssas sector since I joined Dentons back in 2012.
"Advisers are looking to partner with Sipp providers that are committed to the market and are investing to ensure they have the people, systems and processes in place to provide high quality administration.
"That is Dentons in a nutshell, and I look forward to continuing to provide technical help and guidance to advisers."
The Sipp industry is in the process of adapting to the recent Berkeley Burke court ruling, which found a Sipp provider could not rely on an execution-only contract when accepting business without considering the underlying investments and potential client harm.
The ruling could put a number of Sipp providers in danger as evidenced by the soaring number of claims brought against Sipps in relation to failed unregulated investments.
A similar case to Berkeley Burke, Mr Adams versus Carey Pensions, is still awaiting an outcome after being heard in March.
Mr Tilley had previously warned a judgement against Carey could have profound effects on the Sipp market and lead to providers winding up their operations as they are being pursued for past business.
He later clarified: "If Carey do lose, the findings of the judgement will be crucial in determining if it may be the landmark case that some suggest."