Forster struck just eight minutes after coming off the bench following a knee cartilage injury, adding to Darren Currie's superb equaliser that had cancelled out David Vaughan's opener.
Crewe dominated the first half, but were left to rue only going into the break 1-0 up. Their slick passing tore Town apart at times and had they doubled or trebled their lead, the hosts could not have complained.
Ipswich controlled proceedings after the break though, and could even have run out more convincing winners, with Currie's set-pieces causing problems throughout.
Currie had an effort held on the line by Crewe keeper Ben Williams inside the first minute as Ipswich started brightly, but the visitors showed their attacking intent as Steve Jones and Kenny Lunt had shots blocked, before Steve Jones crossed for Eddie Johnson to shoot over.
Williams made a mess of a Jim Magilton 25-yarder but managed to get enough on it to get it around the post, before Steve Jones ran onto a Darren Moss pass but shot well wide when clear.
Richard Naylor twice headed off target from Currie corners before Sam Parkin's flick found strike partner Dean Bowditch, who saw two shots blocked by left-back Moss.
Johnson then got inside Naylor onto a long ball, but embarrassingly shot way wide with the outside of his boot, before Crewe took a deserved lead on 26 minutes.
Vaughan stepped up to curl a free-kick from the right-hand side of the box past a static Lewis Price and into the far corner, with the Town keeper seemingly expecting a cross.
Naylor made a tremendous block to deny Johnson from Lunt's pass before Steve Jones' fierce angled shot forced a good parry out of Price and the forward fired the rebound over.
Price pulled off an excellent one-handed save to deny Johnson from another Lunt ball, and from the ensuing corner, Steve Foster's header was bundled away from the line, with Steve Jones lurking.
Town boss Joe Royle opted to replace top scorer Parkin at half-time, with centre-half Fabian Wilnis coming on and Richard Naylor pushed forward. It proved to be a masterstroke.
Wilnis headed wide when unmarked from a Currie corner before Currie levelled with a sublime 25-yard free-kick into the top corner on 54 minutes.
Jones curled an effort at Price from the edge of the box after a Vaughan pass before Royle sent on Forster for his first action since August 13th with an hour gone, replacing Bowditch.
Steve Jones got clear only to shoot straight at Price before Wilnis got free at another Currie corner, but again failed to hit the target.
Forster showed he had lost none of his sharpness when given a sight of goal from Naylor's flick after 68 minutes as he outpaced Foster and flashed a 20-yard angled strike across Williams to put Ipswich ahead.
He nearly repeated the trick later when he ran onto a Currie ball, escaped Paul Bignot and from a similar position, this time Williams was able to parry the ball away.
Foster sent in a header from a Lunt free-kick late on, but it was straight at Price.
However, Town held on comfortably enough and can now look upwards at the play-off chase, rather than worrying about the drop zone, but brave Crewe will have increased their own relegation fears.