In April 2021, after notifying the chief whip of his intention to resign his position as Parliamentary Under-Secretary of State, he was dismissed by Prime Minister Boris Johnson.

[12] In June 2017, two years after becoming an MP, Mercer published We Were Warriors: One Soldier's Story of Brutal Combat, a memoir of his upbringing and army service, especially his time in Afghanistan.

[14] He said that he entered politics with a view to improving the care of veterans and felt that he was a Conservative because he regarded the "massive welfare state that saps the ambition and drive of a younger generation" as a problem.

[17] Mercer delivered his maiden speech in the House of Commons on 1 June 2015, stating his "main missions" in Parliament to be improving provision for mental health and support for war veterans.

[22] In July 2018 Mercer appeared in Celebrity Hunted, a Channel 4 television programme where participants go on the run and images are released of them so people can try to track them down.

While taking part in the programme he missed the meeting of the Health and Social Care Committee, of which he was a member, shortly before Parliament rose for the summer recess.

The Labour Party parliamentary candidate Charlotte Holloway accused him of neglecting his constituency duties to earn a "staggering" amount of money.

[29] On 8 May 2019, Mercer announced that he would no longer vote for any laws that Theresa May's cabinet presented before Parliament (with the exception of Brexit-related legislation) until new laws were implemented which would end the practice of prosecuting British servicemembers who were deployed to Northern Ireland as part of Operation Banner, stating that "these repeated investigations with no new evidence, the macabre spectacle of elderly veterans being dragged back to Northern Ireland to face those who seek to re-fight the conflict through other means without any protection from a Government who sent them almost 50 years ago, is too much".

[36] He repaid £931.20 in telephony costs and the Compliance Officer made four recommendations to IPSA to improve its guidance to MPs and its processes for checking claims were correct.

Mercer was also tasked by the Prime Minister to focus on ending the legal pursuit of former service personnel, especially those who had served during the Troubles in Northern Ireland.

[46][47] The bill was designed to protect UK military veterans from "unfounded prosecutions", but because it only covered operations conducted outside the United Kingdom (such as in Iraq and Afghanistan), Mercer said it was a "red line" for him that British service members who were deployed to Northern Ireland would be excluded from it.

[56][57] On 27 December 2023, Mercer was accused by his constituency Labour opponent Fred Thomas, of failing to meet his pledge of ending involuntary veterans' homelessness.

[58][59] Mercer rejected the accusation saying "I made a very clear promise on ending veterans sleeping rough because of a lack of provision, this year.

[61][62][63] Thomas told the BBC "I remain unable to discuss much of my service, something which Johnny Mercer – as a former defence minister – is keenly aware of".

Mercer had in January 2020 described, as a government minister to the House of Commons, media allegations of extrajudicial killings by UK Special Forces in the War in Afghanistan as untrue.

Mercer told the inquiry he was angry that the Director Special Forces, the Chief of the General Staff and the Defence Secretary Ben Wallace had "not done their job that was incumbent upon them with their rank and privileges in those organisations" in allowing him to make statements to the Commons which they knew to be untrue.

[65][66] He expressed his later inability to disprove alleged war crimes by the SAS in Afghanistan despite seeking evidence, citing warnings from special forces members.

Evidence was submitted that in an August 2020 letter to Wallace, Mercer had written:[65] That I have been allowed to read out statements to the House of Commons that individuals in strategic appointments in the department knew to be incorrect is completely unacceptable.

Secondarily, I was very cross that I had been allowed to make a statement in the House of Commons in January that year that was clearly incorrect when faced with the evidence that existed within my own department – and for me that was a kind of red line being crossed, in terms of "we're not on the same side here".In the end of March 2024, Mercer was given 10 days to disclose source of claims British troops engaged in war crimes in Afghanistan or face a potential jail term.

When first elected, he slept in East London on his boat several nights a week, stating in The Daily Telegraph at the time that it reduced his expenses costs.

[71][72] On a summer boat trip in 2016, he rescued fellow Conservative MP Scott Mann who jumped into the water having been "ashamed to admit" he could not swim.

[76][77] In August 2020, Mercer sustained a head injury while canoeing on the Tamar, which was treated at Launceston medical centre; he subsequently spent three nights in Derriford hospital, following an infection complication requiring surgery.