Jason Scotland looked to have given Ipswich a fourth straight win with an 82nd-minute goal against the run of play on Saturday.
But Mellis combined with substitute Kieran Trippier to apply the finish in stoppage time that gave Mark Robins' team a deserved share of the spoils.
The clash had offered little in the way of quality and a draw was a fair reflection of two teams stuck in the division's middle reaches.
Barnsley made one change from the side that lost at Leicester last week with Stephen Foster returning from a leg injury to replace Trippier.
Ipswich remained unchanged and they looked to continue their upturn in form from the outset.
They carved out the first chance of the afternoon when Jimmy Bullard found room to fire a shoot just over Luke Steele's bar.
The Tractor Boys' revival has led them away from relegation trouble and just one place behind the hosts before kick-off.
But they were dealt a blow when David Norris was forced off through injury after just 15 minutes.
And it was Barnsley who should have been ahead when Garry O'Connor tested Marton Fulop and wasted the follow-up by firing wide.
That was as good as the first half offered, with only Frank Nouble's weak effort from outside the area presenting the other chance of note.
A scruffy midfield battle was blocking any creative spark from igniting and Damien Delaney and Bullard both went into the referee's notebook in quick succession.
Barnsley came back for the second period with renewed vigour as they probed the Town goal but Fulop again proved to be their scourge.
The Hungarian was at his best to thwart Danny Haynes as his drive was palmed away when it seemed destined for the bottom corner.
Bobby Hassell was next to be frustrated when his fierce long-range effort was held impressively by the 27-year-old.
Connor Wickham responded for Ipswich with a hopeful strike but the away side were finding themselves increasingly pegged back and unable to keep possession.
Further efforts came from Tamas Priskin and Lee Martin but Steele was never seriously troubled by the half-chances as Jewell's side looked unlikely to earn victory.
But with a draw looking inevitable, Scotland - on for Priskin - followed-up on a Lee Martin shot that Steele parried to hand his team the advantage.
However, Mellis was on hand to ensure the game finished level in the 93rd minute, firing into the bottom corner with a tidy finish from Trippier's incisive pass.
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