Reina carried the ball into his own net after after Tomas Rosicky's cross had been bundled on to the post by Marouane Chamakh.
The goal was reward for a late spell of Arsenal pressure but it was harsh on Liverpool, who had Joe Cole sent off just before half-time yet took the lead just after it through David Ngog.
Cole was dismissed on his league debut for a challenge on Arsenal debutant Laurent Koscielny that was clumsy but not malicious.
Koscielny himself also received a red card, but not until moments before the final whistle when he earned a second booking for handball.
The equaliser denied Liverpool what would have been only their fourth victory over the Gunners in their last 19 league meetings, but they will take heart from the performance.
Javier Mascherano was hugely impressive in midfield and Roy Hodgson has clearly stamped his mark on the defence, who looked solid throughout and rarely looked like conceding.
Fernando Torres was left in reserve, and later entered to a huge ovation after pledging his future to the club and recovering from a groin injury, but Arsenal missed two of their bigger stars more.
Cesc Fabregas and Robin van Persie were both said to be lacking match-fitness and while the latter made a late appearance, Fabregas was not even in the squad.
Thomas Vermaelen tested Reina early on with a fierce free-kick but aside from that, the first half was almost devoid of action until the closing moments.
Dirk Kuyt created Liverpool's first opportunity when he turned in the box but Manuel Almunia easily gathered his cross.
New signing Milan Jovanovic cut through the Arsenal midfield with a determined run but Kuyt's final ball again found Almunia.
Arsenal's Samir Nasri had another long-range free-kick blocked and Vermaelen put the rebound wide.
Gael Clichy had Arsenal's first serious chance from open play when he broke clear down the left after 30 minutes and fired a shot narrowly over from outside the box.
Liverpool thought an opportunity was opening up soon after when Almunia failed to claim a corner but referee Martin Atkinson spotted a foul by Daniel Agger on Vermaelen.
The action on the field was tight with neither side giving much away but Arsenal were almost caught when Jovanovic raced onto a long ball.
With Cole walking back from an offside position, the Arsenal defence let the ball bounce and the Serbia international darted into the box, only to blast wide.
The game tsuddenly came alive in the closing minute of the first half.
First Glen Johnson had a shot tipped over by Almunia and Ngog had a header cleared off the line from the resulting corner.
The complexion of the game then changed completely in injury time as Cole was sent off for his lunge at Koscielny.
Koscielny returned to the dressing room on a stretcher at the interval but ran out to start the second period as normal.
He was greeted by boos from Liverpool fans but replays of the tackle suggested that, although the initial decision may have seemed harsh, the law had been applied correctly.
Liverpool seemed determined not to let their numerical disadvantage bother them and made a blistering start to the second half.
They claimed the lead within two minutes of the restart as Mascherano threaded a ball into the box for Ngog and the Frenchman finished unerringly home at the near post.
It was his fourth goal of the season after three in Liverpool's first Europa League qualifying tie.
Not content with that, the Reds pushed forward again and Martin Skrtel headed wide from a dangerous Steven Gerrard free-kick.
Ngog had an even better chance when he leapt to reach another Gerrard set-piece but could not direct his header on target.
Kuyt created another fine chance after good work on the right but no-one could turn in his ball across goal.
Liverpool were reduced to nine men when Agger left the field for treatment after all three substitutes had been used.
Arsenal stepped up the tempo and Tomas Rosicky showed great control with a slaloming run into the area before seeing his shot touched over by Reina.
Maxi Rodriguez then cleared after a desperate goalmouth scramble.
The pressure eventually told in the last minute as Reina failed to grasp the ball after Chamakh hit the post from Rosicky's cross.
Gerrard almost snatched victory as the game went into five minutes of injury time but Almunia kept out his free-kick.
The game ended on more controversy as Koscielny was dismissed.