[14] During his time in the Army, Mercer completed nine tours in Northern Ireland and latterly commanded his battalion in Bosnia and Herzegovina, Canada and Tidworth.
He continued to contribute to radio, television, and print media, mainly on security and defence issues, and in The House magazine in 2009 his profession was stated as Journalist.
Subsequently, he was appointed to a newly created post of Shadow Minister for Homeland Security in June 2003 after Duncan Smith became Conservative leader.
In 2004 he introduced a Private Member's Bill in response to the publicity surrounding the case of Tony Martin that proposed to give householders greater powers when protecting their property from burglary.
Citing his experience of internment in Northern Ireland, as a serving soldier, he highlighted the danger of alienating populations needed for intelligence gathering.
[3] Initially, the Conservative Party stated that the comments were a "private matter",[25] but soon David Cameron forced Mercer to resign as Shadow Minister for Homeland Security.
[27][3] In December 2008, Mercer was appointed as Chairman of the House of Commons Sub-Committee on Counter-Terrorism, to further his work as a member of the Home Affairs Select Committee.
[30] In August 2011, Mercer with his knowledge of Northern Ireland, led the calls for considering the use of water cannon and other robust police tactics to deal with the rioters during the 2011 England riots.
[31] The next day David Cameron approved the use of water cannon after chairing a session of the Cabinet Office Briefing Room, Cobra.
[34] In November 2011, the press reported that Mercer had been taped making disparaging remarks about David Cameron, calling him "despicable" and describing him as an "arse" and "the worst politician in British history since William Gladstone".
[35][36] Mercer resigned the Conservative Party whip on 31 May 2013 following an investigation by The Daily Telegraph and BBC's Panorama caught him on camera accepting payment for lobbying.
[39] On 29 April 2014, after seeing a Commons Select Committee on Standards report into his conduct, due to be published on 1 May, calling for him to be barred from Parliament for six months, Mercer announced he would resign his seat.
[41] The Commons Standards Committee report on Mercer stated that its members were "not aware of a case relating to a sitting MP which has involved such a sustained and pervasive breach of the house's rules on registration, declaration and paid advocacy.
"[42] It stated that Mercer "had deliberately evaded the rules", including engaging in "paid advocacy" and failing to make "any serious attempt to identify his ultimate clients".