Kammala wannan jumla: Mackie ya shiga cikin Marine Corps daga New York a ranar 24 ga Afrilu, 1861. Ayyukansa na farko shi ne tare da Marine Detachment a kan jirgin USS "Savannah". An ci gaba da shi zuwa corporal a ranar 1 ga Maris, 1862. A ranar 1 ga Afrilu, ya fara aiki a kan jirgin yakin Amurka "Galena". Ranar 15 ga watan Mayu, rundunar jiragen ruwa na Union guda biyar, ciki har da "Galena", sun tashi zuwa Kogin James don gwada kariya na babban birnin tarayya, Richmond, Virginia. Bayan ya kai wani juyi a cikin kogin sama da Dutch Gap, rundunar ta ci karo da cikas da ke cikin ruwa da kuma wuta mai karfi daga bataliya biyu na 'yan bindigar Sojan Ruwa da aka sanya a bakin kogin da kuma harin bindiga daga Fort Darling a saman Drewry's Bluff kimanin mil takwas a kasa da Richmond. Mackie ya umurci 12 Marines a kan gun deck. Ƙarƙashin bindigogi na sansanin ya haifar da mummunar lalacewa a kan "Galena" kuma ya tilasta rundunar soja ta koma baya. A lokacin yakin (wanda zai zama sananne a matsayin
Yaƙin Drewry's Bluff), yawancin ma'aikatan jirgin ruwa na "Galena" sun mutu ko suka ji rauni. Mackie ya jagoranci rukuni na Marines wanda ya karbi aiki na bindigogi don sauran yakin. A ranar 1 ga Nuwamba, an ba shi mukamin sajan. A watan Yunin 1863, an sanya shi zuwa.