Ian McParland's side had chalked up back-to-back victories since Sol Campbell's dramatic exit a fortnight ago.
But McParland and Magpies director of football Sven Goran Eriksson saw Hammond strike 15 minutes from time to cancel out Luke Rodgers' 26th-minute opener.
McParland heaped praise on referee Mike Russell - but still blamed him for costing his side their third victory in a row.
The Meadow Lane chief was not happy that Russell failed to give his side a first-half penalty for Michael Townsend's challenge on Rodgers when the visitors were already 1-0 up.
McParland said: "We got the goal and then had a stonewall penalty. It was the only thing the referee got wrong - apart from that he was fantastic but that was a major decision because we would have gone two up.
"The only way I could see Cheltenham scoring was through a long throw or set play, and it was disappointing how we gave the ball away for their goal.
"Lee Ridley's long throw was a right weapon especially if you have big lads like Cheltenham have got.
"But I still think if we had shown a bit more composure we could have got the second goal and won it."
County went into the match on a hot run while Cheltenham were looking for their first win on home soil since the opening day of the season.
But it was the Robins who looked the brighter proposition. And after putting the early setback of Rodgers' goal behind them they warmed to their task, with Townsend and Barry Hayles hitting the woodwork either side of the break.
David Bird also wasted a free header from six yards.
But the Cheltenham pressure from set plays and Ridley's long throws eventually paid off when substitute Julian Alsop flicked on for Hammond to turn sharply and fire in.
According to Robins boss Martin Allen it was the least his side deserved.
Allen said: "It was a fantastic performance right from the start.
"They hit us on the counter-attack but overall I thought we were by far the better team from start to finish and deserved to win. But the chances we've created have got to be taken.
"Against good players you don't get too many and when they come your way they've got to be nailed.
"It's frustrating that we didn't go on and win because we played 90 per cent of that game in their final third.
"We have got to capitalise and create and express ourselves and win these matches.
"It's another point on the board but from two home games this week I'm disappointed to just get one point."
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