The 22-year-old forward has found it difficult to get sufficient playing time in the Premier League and was named in the starting line up for the English Football League (EFL) encounter against Championship side Reading.
Success who showed promise during pre-season spearheaded the attack for the Hornets in the absence of first choice strikers Troy Deeney and Andre Gray.
The Nigeria International had chances come his way and finally converted in the 36th minute to put Watford ahead when he slotted past the keeper.
The Premier League outfit held on to the lead as both sides went for the halftime break.
New boy Quina scored the second goal for Watford as they held for a crucial win which sees Javi Garcia's team advance to the next round.
Success who failed to score in Watford's opening three league matches against Brighton & Hove Albion, Burnley and Crystal Palace has now opened his goal account for the season.
He took to his official Instagram account to post a picture and message celebrating his goal against Reading.
He said, "Thank God for the win tonight and happy to have scored a goal, come on "
Success will hope to continue his good form in front of goal as Watford aim to make it four wins from four games when they host Tottenham Hotspur in their next fixture scheduled for Sunday, September 2 at Vicarage Road.