Joseph E. Widener, the patrician horse owner who built Hialeah Park race track, could not have imagined what would happen in the famous race named for him.

Nor could Mary Russ have envisioned it even a year ago. But the 29-year-old woman today rode Lord Darnley to an upset victory in the $171,500 Widener Handicap, and made history in the process. Never has a female jockey won an event of such magnitude.

Lord Darnley was rightfully ignored at 16 to 1 today, even though this was an unusually weak field for the Widener. The gelding rarely had attempted to run farther than seven furlongs; at 1 1/4 miles he figured to show speed for a while and then collapse.

But Russ managed to restrain her mount and take him back into the middle of the 13-horse field. "There were a lot of speed horses in there," she said, "and I didn't intend to rush him." As the field reached the turn, those speed horses began to weaken, but it was Joanie's Chief who seemed to be making the decisive, winning move. Under jockey Jean-Luc Samyn, he angled off the rail, swooped around the leaders and took command with an eighth of a mile to go.

Behind him, though, Russ had swung Lord Darnley into the middle of the track and let him roll. Lord Darnley caught Joanie's Chief by a head, with Switcheroo, one of the pacesetters, another head behind. Princelet, the favorite, was eighth.

Lord Darnley paid $35.40 to win here; in New York's Off-Track Betting he returned $109.20.

Florida racegoers have learned over the last few months to take Russ very seriously. A late starter, she began her riding career at Hialeah just a year ago, and looked about as bad as most fledgling jockeys do. But by midsummer she was performing respectably, and this winter she reigned as the leading rider at Calder Race Track, only the second woman ever to accomplish such a feat at a major track.

Still, she had been riding mostly against second-rate jockeys at Calder. At Hialeah she has been competing against some of the big names of her chosen profession, and she ranks fifth in the jockey standings.