Marshall is a career police officer who joined the New Zealand Police in 1972, and has worked both in uniform and as a detective with the Criminal Investigation Branch. Within New Zealand, he has served as head of the Hawkes BayArmed Offenders Squad, and Area Commander in Hastings and Auckland City. Internationally he has been posted to the New Zealand diplomatic missions in Canberra from 1998 to 2002,[3] and Washington, D.C. from 2002 to 2004, where he established a New Zealand Police liaison office for counter-terrorism.[4] He was then Assistant Commissioner at the Police National Headquarters in Wellington, before being seconded to the Solomon Islands in February 2007.[5] In May 2008 he became acting Commissioner of Police in the Solomons, and was officially appointed to the position in March 2009. While in the Solomons he was subjected to a home invasion by a group of thirteen people armed with machetes, but was able to defend himself and his wife with a steel ceremonial sword.[6]
In April 2011, Marshall became the Commissioner of the New Zealand Police, and announced he would put a taser in every frontline vehicle, but would not be changing the policy that officers do not ordinarily carry firearms.[7]