[2] Turnbull worked initially in general practice at the Bar, but by 1988 had developed specialities in criminal defence and fraud.

In 1998, he was appointed one of two senior prosecuting counsel in the Lockerbie bombing trial, along with Alastair Campbell QC (now Lord Bracadale).

[3] In February 2001, he was appointed Principal Advocate Depute, resigning to return to private practice in 2006.

He was succeeded as Principal Advocate Depute by John Beckett QC, who would later serve as Solicitor General,[4] being appointed to the Bench later that year.

[5][6] During his time working as Advocate Depute, Alan Turnbull prosecuted Scotland's most difficult criminal cases including that of William Beggs, known as the "Limbs in the Loch" murder.