The visitors survived a barrage of City chances in the first half to take the lead on 29 minutes through Jay Emmanuel-Thomas.
But Ipswich were pegged back by Proschwitz’s first club goal on 73 minutes, set up by fellow summer signing Ahmed Elmohamady.
The German striker’s effort sparked a flurry of chances for the hosts, but it was Proschwitz himself who ensured three points with an injury-time header to give City a second successive win.
City had a point to prove at the KC stadium – having lost their last two home games – and they showed their intentions in the fifth minute when Sone Aluko’s corner was met by Paul McShane, whose shot needed a scrambled clearance from Ipswich.
Aluko was gifted City’s best chance on goal after 16 minutes when defender Danny Higginbotham slipped in the Ipswich box. However, with just keeper Stephen Henderson to beat, Aluko fired well over.
It was City’s game to dominate as Ipswich only had a Lee Martin shot saved easily by keeper Ben Amos to show for their early efforts. Henderson was kept busy at the other end by the Tigers and he needed to be quick to collect Aluko’s through-ball with Jay Simpson rushing on.
Emmanuel-Thomas then gave the visitors the lead against the run of play. A quickly taken free-kick on the right saw the 21-year-old given the chance to jink his way into the box and, with no City defenders closing him down, fire into the bottom corner.
Emmanuel-Thomas almost made it two for the visitors, who were growing in confidence, only for Liam Rosenior to make a last-ditch tackle to deny the shot before Daryl Murphy headed over from a corner just before the break.
City came out firing in the second half with two early chances, as Abdoulaye Faye’s looping header was tipped over by Henderson before Andy Dawson’s long-range effort whizzed narrowly over.
Ipswich retaliated through Martin and Nigel Reo-Coker, both of whom tested Amos with strong shots.
City brought on Aaron McLean just after the hour mark followed by a double substitution with Proschwitz and James Chester taking the field, and the hosts at last found vigour.
Stephen Quinn headed wide from Elmohamady’s cross before Simpson fed Mclean on the right, who shot straight at Henderson.
The pressure yielded rewards on 73 minutes as Elmohamady found space on the right-hand byline to cross low for Proschwitz, who made no mistake from point-blank range.
City should have been ahead on 84 minutes, only for Quinn’s long-range effort to strike the bar before Proschwitz shot hopelessly over from six yards.
But the German made amends in dramatic style by meeting Elmohamady’s cross in almost the final minute of injury time to power the winner past Henderson.