The new protections apply to actions carried out in the course of duty more than a decade ago.
In these cases, there would be a statutory presumption against prosecution for current or former armed forces personnel.
But in exceptional circumstances, such as where compelling new evidence had emerged, the protections could be set aside.
In a statement before the announcement, Ms Mordaunt said: “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.”
She is also expected to repeat the government’s commitment to take up a right to suspend parts of the European Convention on Human Rights before the UK embarks on military operations.
The Ministry of Defence said the suspension, known as “derogation”, would protect British troops from the kind of “persistent” legal claims that followed operations in Iraq and Afghanistan.
The defence secretary is expected to say that lessons from investigating allegations in Iraq and Afghanistan should also be applied to Northern Ireland, although the presumption against prosecution would not apply.
Six former soldiers who served in Northern Ireland during the Troubles are currently facing prosecution.
The cases relate to Daniel Hegarty; Bloody Sunday; John Pat Cunningham; Joe McCann (involving two ex-soldiers); and Aidan McAnespie.
Not all the charges are murder.
The Public Prosecution Service in Northern Ireland said that of 26 so-called legacy cases it has taken decisions on since 2011, 13 related to republicans, eight to loyalists, and five are connected to the Army.
The proceedings have been criticised by some Tory MPs, including former Army officer Johnny Mercer, who earlier this month said he would not co-operate with the government until it ended the prosecutions.