Steve MacManus was the managing editor for Sonic the Comic presumably since the very beginning, although he was first credited in Issue 8 (when the position actually started to be mentioned in the Control Zone). MacManus ceased to be managing editor when Richard Burton gave editorial duties to Deborah Tate and assumed the position himself.
Even when Burton became a consultant, the managing editor position was no longer credited, so it isn't certain whether or not MacManus returned to the job. Even so, his credit returned in Issue 119, once again taking up his original role. It could then be argued that MacManus was the longest serving "contributor" for STC, editing right up to Issue 184 and even to the very last issue. He effectively became the comic's editor from Issue 185 onwards, as the comic became fully reprint and Andy Diggle left the publisher.
MacManus is also the longest serving editor of 2000AD, being in the position from 1978 to 1987. He also wrote many stories for the comic, something he never did for Sonic the Comic.