The visitors began strongly, taking the game to Mosborough and with 10 minutes played Oliver Williamson crashed over to give them the lead.
They moved further ahead a few minutes later when Richard Pettican received the ball in his own 22 and went on a run that evaded everyone to score at the posts.
Scott Houghton added the conversion.
Mosborough struggled to get into the game and Dinnington appeared to have taken a tight grip on proceedings when Pettican scored again, this time running in from halfway, Houghton converting.
The home side threatened for the first time as they came close to scoring but Dinnington were able to keep them out.
Any threat Mosborough appeared to have should have been stifled, as early in the second half Dean McAulay crashed over from 1 yards and Houghton converted to stretch the lead to 26-0.
However, the home side then made changes in their front row and slowly the initiative swung their way as they began to put pressure on at the scrums, rucks and mauls.
Dinnington were a bit slow to recognise that they had a game on their hands and they became fragmented in defence which only boosted Mosborough’s confidence.
Twice the home side scored following line-outs and then a move in the backs created an overlap and a try in the corner.
With 15 minutes to go Mosborough put themselves in with a real chance of a comeback victory as they ran in their fourth try but, Dinnington rallied, regained their shape in defence and held on for, in the end, a hard fought victory.