After Bruno Cesar gave the Portuguese visitors the lead shortly after half-time, a struggling Real -- who won the Champions League when beating city rivals Atletico
in May -- looked set for a shock defeat.
The last time the European champions began the defense of their title with a defeat was when AC Milan were beaten by Ajax Amsterdam in 1994.
Trying to emulate the Italians in a different fashion, by becoming the first team since Milan to defend Europe's top club prize in 1989 and 1990, Real was indebted to Cristiano Ronaldo for its late equalizer.
A minute from time, the three-time world footballer of the year -- who had previously looked off the pace -- drew the eleven-time European champions level with a brilliant free-kick into the top corner.
The goal continued his run of having scored in every competitive game against Sporting, where he made his professional debut as a 17-year-old in 2002, five years after leaving his home in Madeira to join the club's academy.
Fourteen years after scoring on his Sporting debut, the Portuguese bagged his 550th career goal.
He had scored in both his previous appearances against Sporting -- in the 2007/08 group stages for his former side Manchester United -- holding up his hands to apologize to local fans when scoring in Lisbon before celebrating his late free-kick in Manchester.
Now playing against Sporting for the first time as a Real player, Ronaldo celebrated in muted fashion despite the last-gasp nature of his goal.
Real coach Zinedine Zidane will have been delighted by the winner since two of his substitutes combined, as James Rodriguez delivered a sumptuous cross for recent signing Alvaro Morata to head home in the 94th minute.
The victory was significant given that Group F rivals Borussia Dortmund produced a scintillating display as they achieved their biggest European victory when winning 6-0 at Legia Warsaw.
The hosts were the first Polish side to contest a Champions League group match in over two decades but they were soon behind as Mario Gotze scored his first Dortmund goal in 1,250 days following his return from Bayern Munich.
The Germans were 3-0 up after 17 minutes, an unprecedented feat for an away side in the Champions League, and their impressive start sets up a mouthwatering clash when Real visits later this month.
Legia, meanwhile, can expect to hear from UEFA after crowd trouble involving the home fans marred the match.
There was no such trouble in Belgium where surprise English champions Leicester City made the perfect start to their Champions League bow when winning 3-0 at Club Brugges.
Marc Albrighton had the honor of scoring the Foxes' first goal in the competition before Algerian star Riyad Mahrez struck a stunning free-kick and a second-half penalty.
The other match in Group G ended all-square in Portugal as host Porto drew 1-1 with a visiting Copenhagen side, despite the Danes playing the last quarter of the game with ten men.
Tottenham Hotspur has moved its Champions League games to Wembley because of rebuilding work at its regular White Hart Lane Stadium but a record 'home' crowd of 85,011 witnessed Spurs fluff their lines in a 2-1 defeat by French side Monaco.
North London rivals Arsenal struggled while hosting European matches at Wembley and Spurs, who had stated their ambition of bettering the Gunners' record, now have work to do when hosting Bayer Leverkusen and CSKA Moscow.
The two remaining teams in Group E played an entertaining 2-2 draw in Germany.
Juventus may have splashed out nearly $100m on buying striker Gonzalo Higuain from Napoli but he was unable to break the deadlock as the Italians opened Group H with a 0-0 draw at home to Europa League winners Sevilla.
The Argentine had the best chance of the match when heading against the crossbar, as the Spaniards played their first European tie under new coach Jorge Sampaoli.
Elsewhere, Lyon beat Croatia's Dinamo Zagreb 3-0.
For the first time in history, England fielded three sides on a Champions League night after Manchester City's clash with Borussia Moenchengladbach was postponed because of torrential rain 24 hours earlier.
Sergio Aguero showed no signs of being affected by the delay as the Argentine scored his second hat-trick of a month-old season, taking his tally of goals under new coach Pep Guardiola to nine in just five games.
City won 4-0 to ensure they keep pace with Group C leaders Barcelona, who recorded their largest Champions League win when thrashing Celtic 7-0 on Tuesday.
Those goals contributed to the 50 scored on the opening round of Champions League group games.