She was challenged at the hustings, staged by the 1922 Committee of backbench Tories, over her handling of a bill to allow Attorney General Suella Braverman to take maternity leave last year.
Ms Braverman is now one of Ms Mordaunt's leadership rivals and has criticised her over a decision to replace "woman" with "pregnant person" in the legislation.
While the bill was going through the Commons, Tory MPs led by the veteran ex-minister Sir John Hayes tabled amendments to reinstate "mother" and "woman", but they were rejected by the government.
Later, the same amendments were reinstated in the House of Lords after a cross-party campaign including Labour and Conservative peers.
At the 1922's hustings Sir John, a leading supporter of Ms Braverman, challenged Ms Mordaunt on the issue and was later said by colleagues to be "incandescent" at her reply.
"She said she had only had a day to read the bill," one MP present told Sky News. "Anyone who's been a bill minister will tell you you have plenty of time to prepare."