The duo struck within five minutes of each other at the start of the second half to seal Albion's success.
The result was academic as Albion had been promoted even before they kicked-off following Sunderland's failure to beat Wigan.
Bradford, who had won just one of their last eight games, briefly threatened to spoil the party in the first period under the watchful eye of their manager, former Albion star Bryan Robson.
But Albion were not to be denied despite making a lacklustre start.
Bradford almost took advantage of Albion's pedestrian opening when goalkeeper Russell Hoult had to pull off a smart 15th minute save to prevent Lewis Emanuel giving City a shock lead.
Hughes and Horsfield then conspired to waste a host of chances before they finally found their shooting boots.
Horsfield headed Albion ahead with his seventh goal for the club after 55 minutes when he nodded the ball into an empty net after Jason Koumas had seen his chip bounce off the cross bar.
Hughes then sealed the win five minutes later with his 12th goal of the season, cutting in from the right to beat goalkeeper Mark Paston with a low near-post drive.