He quit over legal protections for British troops who fought during the conflict in Northern Ireland.
Speaking to BBC Newsnight Mr Mercer said Boris Johnson had failed to deliver on promises to protect ex-soldiers who served in the Troubles from prosecution.
Mr Mercer said he was sacked by text and has branded Mr Johnson’s government a “cesspit”, adding it was the “most distrustful, awful environment I’ve ever worked in”.
But who is Johnny Mercer? What was his position and role in government? And why is he now so cross?
Who is Johnny Mercer?
Born in Dartford on 17 August 1981, Mr Mercer is the 39-year-old son of a banker and a nurse who worked briefly in the City of London before joining the Royal Artillery after graduating from the Royal Military Academy in Sandhurst in 2003.
He mostly served with the 29th Commando Regiment Royal Artillery and 3rd Regiment Royal Horse Artillery.
He completed three tours of Afghanistan and retired from service in December 2013 with the rank of captain.
He became a Tory MP for Plymouth Moor View in May 2015. During his campaign, he admitted he had not been politically motivated or engaged as a young man - adding the first time he voted was for himself when he ran for office.
In June 2017, Mercer publishedWe Were Warriors: One Soldier’s Story of Brutal Combat, a memoir of his service and time in Afghanistan.
In November 2018, he won Celebrity Hunted - where public figures have to evade capture from a team of expert trackers - beating broadcaster Kay Burley and Love Island stars Chris Hughes and Kem Cetinay.
He became veterans minister in July 2019 after increasing his majority seat by 13,000 votes in the general election.
What was his role, and what was he working on?
On 28 July 2019, Mercer was appointed as Minister for Defence People and Veterans. His responsibility included armed forces personnel and veterans’ welfare.
Mercer was also tasked by Mr Johnson to focus on ending the legal pursuit of former service personnel, especially those who had served during the Troubles in Northern Ireland.
It was an issue he had said he wanted to work on as far back as May 2019.
It was one of Mr Johnson’s campaign pledges to end the“vexatious historical investigations” into service personnel in Northern Ireland.
But the government has struggled to legislate due to sensitivities surrounding this issue.
The MP for Plymouth Moor View has been heavily involved in the Overseas Operations (Service Personnel and Veterans) Bill, which is going through its final stages in Parliament.
The legislation was developed in response to legal claims made after operations in Iraq and Afghanistan, but does not currently cover incidents in Northern Ireland.
Why did he quit?
On Tuesday night Mr Mercer said he quit due to frustration at a lack of progress over the legislation.
Mr Mercer said in the letter to the Prime Minister that not including those who served during the Troubles in the bill was his “red line”, adding: “I am deeply proud of my predecessors who served in Northern Ireland.
“They are not second-class veterans. They deserve the protections of the Overseas Operations Bill like everyone else.
“I made promises on your behalf that we would not leave them behind and would walk through simultaneous legislation for them. No discernible efforts have been made to do so, and I can see no prospect of this changing.
“I have no choice but to leave Government and campaign for them in Parliament.”
On Tuesday, Downing Street said the Prime Minister had accepted Mr Mercer’s resignation as veterans minister. However, the former Army officer said on Twitter that he was “sorry to have been relieved of my responsibilities in Government” amid reports that he was sacked by Mr Johnson - causing mass confusion.
In a letter to the Prime Minister, which was posted on Twitter, Mr Mercer said the Government risks “damaging an already bruised veterans cohort further” with the proposals.
He said he had hoped Mr Johnson’s premiership would “signal a step change in veterans affairs in the UK” and that he had raised his concerns in a face-to-face meeting with him.
“It is with a heavy heart that I am forced to offer you my resignation from your Government,” Mr Mercer said.
“I am of course, desperately sad events have transpired the way they have – I truly have exhausted my efforts and my team to make it work.
“But the truth is politics always was a means to change how this country treats her military veterans, and I remain genuinely appalled by the experiences of some of this nation’s finest people who have served in the Armed Forces.
“I fought and bled alongside them, I’ve been far more fortunate than many of them since, and I have a duty to tell their truth to power.”
What did Boris Johnson say?
Mr Johnson thanked Mr Mercer for his service as a minister following his resignation.
The statement released shortly after 7pm said: “This evening the Prime Minister has accepted the resignation of Johnny Mercer as minister for defence people and veterans.
“He thanks Johnny Mercer for his service as a Government minister since 2019.”
The Prime Minister said in a letter to Mr Mercer that he was “grateful” for his contribution as veterans minister and that he had “made a real difference” to the lives of defence personnel and veterans.
“I am grateful for your service and for everything you have done in the role,” the letter said.
“Alongside helping today’s veterans, you have put in place the solid foundations to build and expand our support for veterans in the future.
“The Office for Veterans’ Affairs has established cross-government leadership to improve the lives of those who have served this country.
“Your determination to fix problems and to make life better for our defence personnel and veterans has made a real difference, and I am grateful for everything that you have done.”
What have the repercussions been?
The main repercussions begun over whether Mr Mercer jumped, or was pushed.
Mr Mercer told the BBC he intended to quit on Wednesday - after the bill was discussed in parliament- but he said he had been gazumped by someone leaking the news to the press on Tuesday - meaning he was asked to resign 24 hours earlier.
The MP said he refused to quit before he planned to - but then suggested he was sacked “by text.”
Mr Mercer told PlymouthLive in an interview: "I gave them the courtesy yesterday of saying I was going to resign because there’s no protection for the Northern Ireland veterans in the bill.
"Number 10 then leaked that to the media and basically tried to put the thumb screws on me to resign. I refused. They sacked me.
"It was via text. It’s pretty pathetic.”
Mr Mercer then went on to blast the Government as “the most distrustful, awful environment I’ve ever worked in” where “almost nobody tells the truth.”
Mr Mercer said on Wednesday that “nothing has been done” over the “gross betrayal of people who signed up to serve in the military”, as he gave a damning account of Mr Johnson’s Government.
“This is the most distrustful, awful environment I’ve ever worked in, in Government. Almost nobody tells the truth, is what I’ve worked out over the last 36 hours,” he told Times Radio.
The MP said he was “made to feel like I’m the last man in the room who’s willing to fulfil our manifesto commitments”, as he described politics as a “cesspit”.
“You know, I find this place has taught me a lot about the Government, a lot about my colleagues: let’s say shooting straight is not one of their finest qualities,” he added.
Former Scots Guards captain Leo Docherty was appointed as Mr Mercer’s successor yesterday.