In 1994 he was invited to become an HM Treasury counsel,[1][2] and was appointed First Senior Treasury Counsel from 2006 to 2008.[1][2] During this time he advised and acted in: complex serious fraud; corruption in public and private bodies; third party disclosure; trial secrecy; press freedom issue; admissibility and gathering of foreign evidence, including intercept and official secrets act; advice on prosecuting members of political parties for donation offences; advice on criminal law issues connected to the alleged illegality of the Iraq War.[1][2] He has also worked for clients with regards professional practise and disciplinary issues in front of the General Medical Council.[1][2]
In 2012, Ellison successfully led and secured the first convictions of any of Stephen Lawrence's murderers, through the convictions for murder of Gary Dobson and David Norris. Despite inquiries by both Scotland Yard and the Independent Police Complaints Commission ruling that there was no new evidence to warrant further investigation, after discussions with Dame Doreen Lawrence, Home SecretaryTheresa May commissioned Ellison to review Scotland Yard's investigations into alleged police corruption.[3]
Ellison was appointed Queen's Counsel in 2008.[1][2] Ellison is a past chairman of Bar Council's remuneration and terms of work committee, and a Bar Council member of the Criminal Justice Council.[1][2]