Horsfield blasted home what proved to be the winner after picking up a throughball from Ian Breckin to claim his second goal in consecutive matches on his home debut and endear himself to the Wigan contingent in the 13,000 crowd.
And no-one can deny Wigan their day after a pulsating performance that took them to the top of the table.
After a slow start they produced some tremendous football and where unlucky not to go in front on more than one occasion as they overpowered a West Brom side that has won its five previous games.
But Horsfield's strike ensured Wigan have now equalled that run although manager Paul Jewell is keeping his feet firmly on the ground.
He said: "We showed two sides of our team tonight. In the first half we played some great football at a high tempo, but in the second half we had to dig in and we defended superbly.
"I am not getting carried away after seven games, but it's plain to see that we are making progress."
Wigan: Filan, Eaden, Jackson, Breckin, Kennedy, Liddell, Bullard, Dinning, McCulloch, Ellington, Horsfield. Subs: Walsh, Teale, Mitchell, Burchill, Roberts.
West Brom: Hoult, Haas, Gaardsoe, Volmer, Sigurdsson, Clement, Gregan, O'Connor, Koumas, Sakiri, Hulse. Subs: Murphy, N'Dour, Hughes, Johnson, Dichio.
Referee: M Jones (Cheshire).
Attendance: 12,874