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But the new measures won't apply to cases in Northern Ireland.
Ms Mordaunt wants new a new law to stop veterans and serving personnel being subject to repeated investigations.
It would mean current and former personnel would only be prosecuted in "exceptional circumstances" over offences committed in the course of duty more than 10 years ago.
The proposal is subject to consultation.
Ms Mordaunt, who is expected to unveil the new measures within days, said: "We all owe a huge debt of gratitude to our Armed Forces who put their lives on the line to protect our freedom and security.
"It is high time that we change the system and provide the right legal protections to make sure the decisions our service personnel take in the battlefield will not lead to repeated or unfair investigations down the line."
Harry Wragg, who organised the Rolling Thunder protest in April in opposition to the prosecution of a British soldier over Bloody Sunday, believes crown immunity is the only protection for soldiers.
He said in reaction to the Defence Secretary's announcement: "I don't think this really matters, crown immunity is the only type of protection these soldiers need, anything else is irrelevant.
"You can't prosecute someone from 46 years ago and expect a fair trial, you can't give immunity to the IRA and not everyone else.
"Terrorists are walking away while soldiers are being punished for doing their job."
Reacting to the measures not applying for cases in Northern Ireland he added: "I can't believe that, it's unbelievable, it should apply to all, that's simple.
"Northern Ireland is the issue that caused it all".
The Cabinet minister is also expected to reaffirm her commitment to derogating from the European Convention on Human Rights (ECHR) before the UK embarks on significant military operations.
Conservative MP Johnny Mercer, who withdrew support for the Government over the stance on historical prosecutions of UK veterans, told Sky News on Wednesday: "It's unfair to all sides, and the only people who are enjoying this process and making something are legal teams.
"We've seen the crass abuses of this process in Iraq and we've got to stand up for these people and not get mired in this nonsense of asking for an amnesty for people who serve, because no-one is asking for that."
He added: "Northern Ireland represents a particular challenge, the way that conflict was framed in the time, it was not classed as war even though it had many traits of wartime activity.
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"We need to redouble our efforts and see what we can do to apply legislation to stop this process taking place. There's a lot of politics and work here and you often hear what people are prepared to accept or not prepared to accept in Northern Ireland, these are our people, these are our veterans who served our nation."
In 2016, Theresa May announced that the Government will adopt a presumption that it will take advantage of a right to suspend aspects of the ECHR at times of war.
Mrs May said at the time that the move should end an "industry of vexatious claims" which has seen veterans of Iraq and Afghanistan pursued through the courts over alleged mistreatment of combatants and prisoners over a decade after the supposed events took place.
Separately, Ms Mordaunt is due to address the First Sea Lord's Sea Power Conference on Wednesday morning.
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