Johannes Flum gave the hosts a deserved lead after half an hour, but Pierre-Michel Lasogga stunned the Waldstadion into silence on the brink of half-time as he brought Hamburg level.
Marco Russ then restored the Eagles' advantage in the 54th minute, but Jansen's deflected effort denied them at the death.
Frankfurt showed no mercy on the new Hamburg coach as they gave him a tough welcome back to the Bundesliga, Stefan Aigner going close to scoring in the 18th minute when his header bounced up and hit the crossbar.
It was only a matter of time before the hosts took the lead, making it look easy in the 31st minute as Flum scored from close range after being left unmarked at the far post.
Rafael van der Vaart curled a free-kick just around the far post, but Frankfurt continued to pose the greater threat with Rene Adler tipping Vaclav Kadlec's long-range shot over.
Adler was heavily involved in keeping Hamburg in the game before Lasogga levelled deep into first-half stoppage time with a header following a corner.
Frankfurt saw a goal ruled out when Russ' shot from the edge of the penalty area was fumbled by Adler, who was grateful to see Aigner turn the ball in from an offside position.
Their pressure was made to count soon after, though, when Russ put them in front again, finishing off an excellent team move from close range with Adler unable to reach his shot.
Adler came to Hamburg's rescue several more times before Frankfurt started to play the clock down.
There were still two minutes to go when Jansen struck an equaliser, thanks to a wicked deflection which gave Kevin Trapp no chance.