Kammala jumla mai zuwa: Rennell ya shiga rundunar sojan ruwa ta Royal a matsayin mai kula da jirgin ruwa a lokacin da yake da shekaru goma sha huɗu a karkashin Kyaftin Hyde Parker, wanda ke cikin tsarin yin amfani da jirgin ruwa (wanda aka kaddamar a watan Oktoba 1757) a farkon Yakin Shekaru Bakwai. Ya halarci taron
An kai hari kan Cherbourg (1758), kuma a cikin mummunan yakin Saint Cast a wannan shekarar. A shekara ta 1760, ya fita zuwa Gabashin Indiya, kuma ya yi aiki a karkashin Kyaftin Hyde Parker a cikin shekaru uku masu zuwa, lokacin da ya ga wasu ayyuka masu aiki, ciki har da binciken da aka yanke a Pondicherry. Ba da daɗewa ba ya mallaki ka'idar da aikin binciken teku, kuma, saboda kwarewarsa a wannan batun, Parker ya ba da sabis ga Kamfanin Gabashin Indiya. Ya yi hidima na shekara guda a ɗaya cikin jiragen ruwa na kamfanin da ke zuwa Philippines, da nufin "kafa sababbin reshe na kasuwanci da 'yan ƙasar da ke tsakanin". Rennell ya kasance tare da masanin ilimin ruwa Alexander Dalrymple kuma ya zana taswira da tsare-tsaren tashoshin jiragen ruwa da yawa a kan tafiye-tafiye a cikin jirgin ruwa "Cuddalore" (1759<unk>62), "London" (1762-63) da "Neptune" (1763<unk>64).