The match was locked at 14-14 on Saturday night after Manly scored three second-half tries before halfback Daly Cherry-Evans conceded a match-winning penalty late in the clash.
Cherry-Evans booted a goal line drop-out over the sideline with four minutes left on the clock.
Aidan Sezer calmly kicked the penalty goal from right in front.
In a heart-stopping final few minutes, Manly attacked the Titans' left-hand side before a Jamie Lyon pass was spilled over the sideline by winger David Williams.
Manly interchange forward Richie Fa'aoso's 36th-minute shoulder charge which wiped Harrison out of the game proved a huge turning point following an epic first-half defensive battle.
A distressed Harrison was ferried from the field with his neck in a brace on the back of a medicab with the crowd chanting "send him off, send him off".
Fa'aoso was placed on report and could be the first player suspended under the game's new shoulder charge rules outlawing the dangerous tackle.
Sezer thumped over the penalty for a 2-0 halftime lead.
The home side started the second half positively and led 14-0 after converted tries to big Ryan James (46th minute) and winger David Mead five minutes later.
However, the Titans let Manly cross twice in the space of three minutes.
Winger Jorge Taufua dived over in the left corner in the 57th minute before replacement Tom Symonds muscled his way over in the 60th after Matt Ballin, Kieran Foran and Cherry-Evans swept the ball to the right side.
Skipper Jamie Lyon missed both difficult conversions as Manly pulled back to 14-8 with 20 minutes remaining.
When Taufua, who scored a hat-trick against Newcastle last weekend, crossed for his second and Lyon converted from touch, the match was deadlocked at 14-14.
Sezer had two shots at a field goal before kicking his game-winning penalty.