The beaten Division 2B finalists looked in danger of morale crushing back to back defeats when they trailed by two with four minutes to go.
But points from free-taker Christy Moorehouse and George O’Brien levelled it before McCormack struck at the death for the winner.
Wicklow’s 0-17 to 0-16 win was cruel on London who were three clear at half-time.
The Exiles retained a 0-16 to 0-14 advantage in the 66th minute but ultimately failed to score in the final 13 minutes of the game.
Wicklow take on Kildare next weekend in Round 2A while London have a Round 2B clash with Roscommon, who were also beaten in their opening game.
The Rossies went down by 2-22 to 1-20 to Down on Saturday in Ballycran.
Roscommon led 1-13 to 1-10 at half-time and surged five points clear after 40 minutes when Robbie Fallon converted a free.
But ultra experienced Down sub Gareth ‘Magic’ Johnson turned the game back in the Mourne men’s favour, scoring a point and making excellent catches and distributions.
Johnson also set up Eoghan Sands for Down’s second goal and won several frees for Paul Sheehan to convert.
Sheehan finished with 15 points while Malachy Magee netted too and claimed 1-2.
Neighbours Antrim needed extra-time at Netwatch Cullen Park to see off Carlow, 3-20 to 2-22.
Neil McManus hit 1-13 and starred for the Saffrons who also had goalscorers in Ciaran Clarke and Niall McKenna.
Antrim had to come from four down at half-time to force extra-time at 2-16 apiece and came good in the extra period despite 1-16 overall from home hero Denis Murphy.
Next up for Ulster champions and beaten 2016 Ring Cup finalists Antrim is an all Ulster Round 2A clash with Down on Saturday.
Kildare, the 2014 champions, held out for a 0-17 to 0-16 win over 14-man Mayo in Ballina.
Relegation from Division 2B of the Allianz league didn’t affect Mayo who, with dual star Keith Higgins in action, fought back from a 0-10 to 0-6 half-time deficit, and the loss of David Kenny to a red card, to cut the gap to one late on.
Higgins had an opportunity to tie it up in the 75th minute but agonisingly fired a long range free wide.
Limerick native and free-taker David Reidy top scored for his adopted Kildare with 0-13.
Meanwhile, there were wins for Donegal, Tyrone and Derry in the Nicky Rackard Cup.
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