The 30-year-old striker is now fully recovered from knee surgery and found the target seconds before the break, before killing off Bristol City on 69 minutes after Liam Fontaine had put through his own goal.
The Sheffield side, looking for a hat-trick of wins after a barren spell, had struggled to break down Bristol's resolute defence, with Stephen Quinn and Billy Sharp trying their luck with long-range shots which flew over the target.
United almost found a way through in the 23rd minute when Beattie passed inside to Quinn, whose fierce strike from 16 yards was smothered by the diving Adriano Basso.
Bristol were almost caught out when Gary Speed found space on the left flank, but his teasing cross dropped onto the roof of the net.
The Ashton Gate side finally managed to penetrate the United defence in the 42nd minute as Dele Adebola picked out defender Louis Carey, but he fired over as he burst into the area.
Sharp linked neatly with David Cotterill whose low strike stopped by full-back Bradley Orr, before United broke the deadlock seconds before the break when Beattie met a Cotterill corner with the sweetest of volleys home from 12 yards.
Basso pulled off an eye-catching stop diving to push away Beattie's stinging drive before United increased their lead in the 56th minute.
Fontaine headed into his own net under pressure from Matt Kilgallon as he tried to keep out a Quinn free-kick.
Beattie added the third with a textbook header from a Cotterill cross, before helping his defence by clearing a Fontaine header off the line in the dying minutes.