Rubuta ci gaba ga wannan sakin layi - A farkon yakin basasa, Mansfield ya umurci Ma'aikatar Washington (Afrilu 27 - Agusta 17, 1861), kuma an ci gaba da shi zuwa brigadier janar a ranar 6 ga Mayu, 1861. Janar-in-chief Winfield Scott ya ba da shawarar Mansfield don jagorancin sojojin sa kai da aka kafa a Washington DC a wannan bazara, amma umurnin ya tafi Irvin McDowell maimakon. Ba a dauki Mansfield a matsayin mafi kyawun zaɓi ba saboda shekarunsa da shakku da sojojin sa kai da kuma turawar gwamnati don saurin tafiyar da Richmond wanda zai kawo karshen yakin a cikin 'yan watanni, kuma shi ma ba shi da mai tallafawa siyasa a Washington. Bayan nasarar da kungiyar ta yi a Bull Run, George McClellan ya zama kwamandan yakin basasa a Gabas ta Gabas kuma Mansfield ya yi fatan samun kwamandan sojojin. McClellan kuma bai ba shi umurni a cikin sabon rundunar sojojin Potomac ba. Mansfield ya kasance a Hatteras Inlet, North Carolina
, a watan Oktoba, bayan yakin da Benjamin Butler ya yi a watan Agusta. Ya kasance kwamandan brigade a Ma'aikatar Virginia daga Maris zuwa Yuni 1862. Ayyukansa kawai na yaƙi a wannan lokacin shi ne harbi na batura na bakin teku daga Hampton Roads a kan "CSS Virginia" a cikin yakin da ake yi da "USS Monitor" a ranar 9 ga Maris, 1862. Har zuwa lokacin kaka na 1862, Mansfield ya umurci Sashen Suffolk na VII Corps na Ma'aikatar Virginia a kusa da Suffolk.