Ka cika wannan jumla: Ya koma Ingila a shekara ta 1642, kuma ya yi aiki na ɗan lokaci a rundunar majalisar, inda ya umurci dakarun doki na mahaifinsa mai rashin lafiya. A watan Afrilu na 1643, bayan mutuwar mahaifinsa, ya yi watsi da sarki a Oxford. Yanzu Earl na Peterborough, ya shiga Cavaliers kuma ya yi yaƙi a yaƙe-yaƙe na Bristol, Gloucester da
Newbury a shekara ta 1643. A Newbury ya ji rauni a hannu da cinya, kuma an harbe dokinsa a ƙarƙashinsa. A matsayin kwamandan rundunar da aka tara a kansa ya yi aiki a yammacin lokacin bazara da hunturu, a Cropredy Bridge da Lostwithiel a 1644. A cikin Disamba 1644, ya auri Lady Penelope O'Brien (gidan 'yar 5th Earl na Thomond) kuma suna da' ya'ya mata biyu. Ya kasance a Faransa a lokacin matakai na karshe na gwagwarmaya. A shekara ta 1646 ya koma Ingila kuma ya haɗu da dukiyarsa. Ganawa ta sirri tare da Charles I yayin da ya wuce ta Ampthill zuwa Hampton Court, a lokacin rani na 1647, ya sa ya yi ƙoƙari na ƙarshe a madadin sarki, kuma a watan Yulin 1648 ya haɗu da George Villiers, Duke na 2 na Buckingham da Henry Rich, 1st Earl na Holland a ɗaga matsayin sarauta a Dorking. Shirin shine a kwace Reigate, amma an hana shi a cikin wannan, an kori 'yan tawayen zuwa Kingston, kuma daga karshe sojojin majalisa suka warwatse a unguwar Harrow (7 ga Yuli). Mordaunt ya ji rauni sosai, amma ya tsere zuwa Antwerp, kuma a shekara mai zuwa ya koma Ingila kuma ya sake haɗuwa da dukiyarsa (Mayu 1649).