He also said Ms Truss had broken a promise not to downgrade his then role of Minister for Veterans’ Affairs which he claimed she made to him during the leadership campaign.
The MP for Plymouth Moor View was dismissed from the ministerial role not long after Ms Truss became Prime Minister, shifting the veterans’ affairs brief in that of the Minister for Armed Forces and Veterans.
On his sacking, Mr Mercer in an interview with Alastair Campbell for Men’s Health magazine, said: “When this happened it was such a gut punch. I remember Liz kind of laughing while she did it.
“She was giggling and looking at the floor. I was just thinking, ‘I’m a mug for believing these guys. I’ve given seven years of my life to this, put my family through the mill for it, and now you are laughing at me.’
Asked if Ms Truss could win him back, Mr Mercer said: “Well, she has changed her views a lot over the years. There is an extraordinary arrogance here. You come in and within 28 days, you’ve gifted a 33-point lead to the opposition.
“It’s hard to get people to do what you want them to with that on your record. Almost impossible.”
He added he would find it hard to answer whether the country was better off with Labour in Government or one led by Ms Truss.
Meanwhile, Senior Tory MP Tobias Ellwood, who had lost the Whip under former Prime Minster Boris Johnson, said the Government had conducted an “experiment” with the economy that did not go well with its mini-budget.
He said: “It’s hard to get people to do what you want them to with that on your record. Almost impossible.”
But he said there was a “calmer Westminster” on Tuesday compared to the past few days and weeks, as “we have got back to understanding what the Conservatives usually do well”.
Mr Ellwood told Channel 4 News: “There’s no doubt about it, it’s been an experiment we’ve conducted with the economy and it’s not gone well. And there’s a recognition that we now need to reboot, we need to reset, we need to regroup.”
He added: “The mechanics in which we chose our leader, I think, is the core problem that (meant) we ended up where we are today.
“If you only ask our membership which direction they want to go, you’re going to get particular answers. And what was clear is that growth productivity has been a problem in the UK going back to 2008.
“Liz Truss presented a package of measures which clearly were too radical, they didn’t include an understanding of the international headwinds.”
Ms Truss today survived a meeting of her Cabinet without any ministers calling on her to quit, but Tory members and voters appear to have turned on her.
A snap poll of party members released on Tuesday saw more than half respond that she should resign and 83 percent say she was doing a bad job.
Downing Street insisted the Cabinet fully supported the Prime Minister and stressed that Ms Truss was focused on the challenges facing the country rather than party infighting.