The record has to be from 1993, which was a historically hot summer, record number of 90 degree days and record number of 100 degree days, 3 in a row at Central Park, 2 at JFK and a mind boggling 9 at Newark including 5 in a row! Back in the 90s we had hot summers much more consistently than we do now.
Check out that super heat wave of 7 straight days of 90+ in early July 1993, you simply don't see this kind of thing anymore. I think the only other two times since then we had such lengthy heatwaves was in 1999 and 2002 -- also very memorable summers !