A Kevin Horlock goal on the half-hour had given Swindon a lead they scarcely deserved but from that moment they looked capable of holding off a rather desperate Southampton side, and nearly did so, only being thwarted by Gordon Watson's header 13 minutes from time.
The Southampton manager, Dave Merrington, said Watson was his side's best player on the day, but, he admitted: "I had a problem with Gordon in the past when his approach was not right. Some of our fans wondered why I was not playing him, but you get some players who don't want to train during the week and think they can turn it on just on a Saturday - they can't.
"You must work hard, and we have worked to change his approach. He is working as hard as anyone now, deserves his place - and you saw the result today." The teams will replay a week next Wednesday, and both must be hoping for better performances than they delivered yesterday. In fairness, having played adrenalin-consuming Cup-ties earlier in the week, both teams looked jaded and leg weary from the start of this tie. Southampton certainly set out in less cavalier fashion than they had done at Crewe, whose second-half fightback had emphatically marked the Saints' card about the likely form of a side at the top of the Second Division.
Indeed, Southampton had more or less achieved their initial objective of dominating midfield when they conceded a goal out of nothing. Swindon had just forced their first corner to excite the home crowd, and within a few moments they had even more to cheer about.
Paul Allen, a veteran of two FA Cup finals with West Ham and Spurs, broke smartly from midfield and his crisp pass to Peter Thorne was matched by the centre-forward's flick into Horlock's path. Thoroughly left-footed, Horlock nevertheless struck a confident right-foot shot past Dave Beasant into the far corner.
Southampton had enjoyed a good deal of possession by this stage but had achieved very little penetration - a Tommy Widdrington shot straight at Fraser Digby, and a Neil Shipperley cross along the face of the goal, were their best efforts of the half.
Now they were obliged to switch from careful containment to judicious attack, in an attempt to draw level. But their efforts, certainly in the period after the interval seemed tinged with desperation once Matthew Le Tissier had fluffed a clear chance with a weak left-foot shot.
Conversely, Swindon grew in confidence with Ian Culverhouse in defiant form at the back, and Ty Gooden and Ling breaking well from midfield. Only Simon Charlton's downward header, which bounced up to clear the bar, came close as Swindon scented the sixth round for the first time in 26 years.
But then Wayne Allison blazed over the Southampton bar with a clear sight of goal and this ominous miss seemed to take the fight out of Swindon.
Moments later Southampton's second successive corner found Watson's head and the ball ricocheted down from the bar and behind Culverhouse before crossing the line for the equalising goal.
Afterwards, McMahon rued his team's inability to hold on to the lead, but his own fateful absence from the pitch was probably the main reason why Swindon did not complete a historic week.
McMahon admitted: "It wasn't a brilliant performance by us, but Southampton were relieved to get the draw. With four minutes to go they were asking: 'How long left?'"