Fletcher's effort three minutes from the end of the first half proved the difference in an even encounter at St Mary's.
The hosts came close in the opening seconds as Kelvin Davis superbly denied Stephane Sessegnon from 25 yards. John O'Shea nodded over from the resulting corner, much to the relief of Southampton.
Jason Puncheon saw a threatening prod goalwards denied by Carlos Cuellar and, after a dangerous Morgan Schneiderlin free-kick was dealt with, Rickie Lambert was inches away from the opener.
Sebastian Larsson was denied by Davis when Sunderland returned to the attack. Both sides were playing cautiously in a bid to find the opener, with the defences dealing with any potential threats.
First Cuellar denied Lambert as he prepared to break through on goal and then Maya Yoshida did well to stop Fletcher meeting a driven Craig Gardner cross.
Puncheon saw a strike blocked by O'Shea as Saints looked to end the half on top, only for Fletcher to break the deadlock. Adam Johnson's low cross from the left was played on by Sessegnon for the Scotland international to sweep home.
Referee Howard Webb waved away Mayuka's penalty appeals as the first half came to a close, with Lambert firing just over 30 seconds after the restart.
Sunderland could have had a second had somebody connected with a drilled Gardner effort before Johnson fired over. Puncheon was dispossessed as he ran through on goal and Steven Davis saw a shot blocked by Jack Colback.
Davis saved an audacious Gardner free-kick from range, before Guly do Prado came on for Puncheon in a final throw of the dice by the hosts. However, Southampton had a late penalty appeal waved away and Do Prado saw a stoppage-time shot blocked.