The substitute nodded in to clinch a 2-2 draw and maintain the Seagulls’ assault on the top five.
It is still the first time the Foxes have earned a point after falling behind this season.
The Seagulls missed the chance to move into the Premier League’s top five – but climbed to sixth – while Leicester moved up to 14th in a congested mid-table.
Boss Brendan Rodgers, somewhat apologetically conceded in the week, time is catching up with his star man who has scored just once in the league this season.
Even after netting 17 times in 33 outings last campaign, there had been little to suggest this term the former England man would be able to roll back the years many more times.
A heavy touch and an early clatter from behind by Pervis Estupinan highlighted the forward is simply just slowing down.
Goalscoring has not necessarily been an issue for the Foxes – their tally of 28 is the best in the bottom half – but they need to find more to continue to disrupt the market as they previously managed.
Even with Brighton’s high-risk slow build-up from the back, the hosts were unable to find the gaps as the Seagulls bossed possession.
Yet it took until the 24th minute for Roberto De Zerbi’s side to create an opening, and even then it came from a corner, when Jan Paul Van Hecke headed Pascal Gross’ delivery over, although Danny Ward’s touch had altered the flight.
Brighton, though, did not have to wait long for an opener which arrived four minutes later.
The Japan international was then given plenty of time to tease Timothy Castagne, cut inside and brilliantly curl the ball into the top corner from 20 yards.
It was a deserved lead but one they only held for nine minutes. The Seagulls had been comfortable, content to knock the ball around at their own pace, but were undone when the Foxes burst into life from nowhere.
Kiernan Dewsbury-Hall swept the ball wide to Barnes, who darted inside and his shot was blocked by Joel Veltman.
A leveller was a surprise given the level of control Brighton had and the second half continued in a similar pattern with the Seagulls furious not to be awarded a penalty when Welbeck was clipped by Thomas.
It appeared a foul, with the Seagulls surrounding referee Thomas Bramall and De Zerbi was booked on the touchline for his protests.
Despite their frustrations the visitors still created a glorious opening minutes later thanks to more good work from Mitoma.
The forward skipped down the left and teed up the unmarked March, who slipped and ballooned his effort over from 12 yards.
It proved crucial and Brighton’s mood got worse when the Foxes completed the turnaround after 63 minutes.
Albrighton swung in a corner, which was touched on by Thomas, for Barnes to rifle in at the far post.
Leicester were able to use fit-again James Maddison off the bench for the first time in two months but still needed Ward to thwart March and Mac Allister.
Yet there were unable to hold on and Ferguson grabbed the leveller with two minutes to go when he glanced Estupinan’s cross in off the post.