Defender Buxton deflected home a shot from fellow defender Richard Keogh in the 66th minute with the score at 1-1 and Derby withstood a late barrage before substitute Davies finished superbly on the break in the fourth minute of time added on.
Leeds midfielder Green cancelled out Sammon's 15th-minute opener for Derby on the stroke of half-time and after Buxton's close-range effort the home side held on with Davies rubber-stamping their sixth league win of the season at Pride Park.
Leeds, who had won their previous three matches, hit the post via Tom Lees' late header and will feel aggrieved not to have been awarded a last-minute penalty when Green went tumbling under a challenge from Will Hughes.
Derby, unchanged from the side that lost 4-1 at Leicester last week, took the lead when Hughes pulled the ball back across the face of goal from the left into the path of Sammon, who drove home a low shot from 12 yards for his fifth goal of the season.
Leeds goalkeeper Paddy Kenny pulled off a fine fingertip save to keep out Theo Robinson's snapshot from close range in the 24th minute and denied the same player from 30 yards soon after as Derby took control.
Leeds, who recalled winger Ryan Hall in place of the injured Jerome Thomas, did not threaten Adam Legzdins in the Derby goal again until midfielder Michael Tonge's 25-yard effort, which was blocked by Derby defender Richard Keogh in the 36th minute.
But the visitors went in level at the break thanks to Green's strike against his former club on the stroke of half-time. Leeds striker Luciano Becchio played the ball inside the penalty area and Green applied a neat low finish into the bottom corner.
Leeds pegged the hosts back in their own half after the restart. McCormack's shot from inside the area was well held by Legzdins and the Scot pulled another effort from a similar position wide.
Derby midfielder Hughes tricked his way into the area before unleashing a shot destined for the top corner, only for Kenny to save at full stretch.
Robinson's goalbound header was blocked by Leeds defender Lees, but the visitors failed to clear the resulting corner and Keogh's low shot was sidefooted home by Buxton.
Leeds went close to a rapid equaliser when David Norris connected sweetly with the ball from 20 yards, but Legzdins was equal to it, and when Lees beat the Rams keeper with a header from substitute El-Hadji Diouf's superb cross, the ball came back off the inside of the post and was hacked clear.
Diouf's thumping drive from 20 yards then whistled past the post as Leeds pushed for an equaliser before Davies, an 82nd-minute replacement for Robinson, curled home a fine 94th-minute effort for Derby after a swift counter-attack.