Ivan Ramis headed in the opener for Wigan from Jordi Gomez's cross, but Andy King levelled four minutes before the break as the teams were tied 1-1 at half-time.
Wigan regained the lead in the 62nd minute through Rob Kiernan, again from a Gomez assist, and they looked to be on course for their first win in three games until Hammond headed in the equaliser three minutes before the final whistle.
Elsewhere, Jordan Rhodes scored twice for Blackburn but they had to settle for a point as they drew 3-3 against Brighton at Ewood Park.
David Rodriguez put Brighton ahead in the 17th minute before Rhodes levelled and Tom Cairney gave the home side a 2-1 lead at the break.
Gordon Greer equalised for the Seagulls in the 79th minute, but Blackburn fans thought Rhodes had nicked a win from the penalty spot four minutes from the final whistle after he converted following a foul by Greer.
However, Brighton had the final word and claimed a point in their pursuit of the play-off places as Dale Stephens headed another equaliser in the 88th minute.
In the night's other game, Ross McCormack missed a last-minute penalty for Leeds as they fell to a 1-0 defeat at home to strugglers Charlton.
Reza Ghoochannejhad scored the only goal of the game for the visitors in the 55th minute.