Sey worked for the United Nations (UN) and the Economic Community of West African States (ECOWAS) in Liberia, and also as a legal adviser in the Office of the Special Representative of the Executive Secretary of ECOWAS.[1] In February 2007, Sey had been appointed to the High Court of Sierra Leone as part of the British Council's Justice Sector Development Programme. During her time in Sierra Leone, she convicted and sentenced a government minister in a corruption case to five years.[2] She served until November 2010, when she was sent by the Commonwealth Secretariat to serve as a judge in Swaziland. She served a two-year contract before being reassigned.[3]
In late 2015, Sey oversaw a landmark corruption case in Vanuatu where 16 members of parliament were put on trial for corruption. Sey ruled that all 16 would stand trial under the Penal Code and the Leadership Code Act, despite the fact no one had been tried under the Leadership Code Act in Vanuatu since it was enacted in 1998. Among those on trial was the incumbent Deputy Prime Minister, Moana Carcasses Kalosil, four other cabinet ministers, the Speaker of Parliament, Marcellino Pipite, and two parliamentary secretaries.[4] During the case, Sey warned Carcasses against threatening witnesses after one was forced to report his threats to the police. Sey warned Carcasses that his bail conditions would be revoked.[5]
In the judgement, 14 were convicted, with one having pleaded guilty prior to the case and one other being acquitted. Sey said to the convicts, "You were given power and authority. With power and authority, comes an obligation of trust. You betrayed that trust and in the cause of doing that you undermined the very institution that it was your duty to uphold."[6] An appeals case has since found that Sey "was fully justified" in her ruling.[7]
In 2017, Sey left as a Judge of the Supreme Court of Vanuatu.[8]