Her dam Drumette showed limited racing ability but was descended from My Lady Love, a half-sister to the Kentucky Oaks Winner Heavenly Cause.

[5] The filly made her first appearance on dirt in the Rags to Riches Stakes at Churchill Downs on October 29 and started the 3.4 : 1 second favorite against nine opponents.

[6] For her final start of the season, Monomoy Girl was stepped up to Grade II class for the Golden Rod Stakes over eight and a half furlongs at the same track on November 25.

[8] In the Grade I Ashland Stakes at Keeneland Race Course on April 7, Monomoy Girl started as odds-on favorite against six opponents and never looked in any danger of defeat.

In the stretch, the race developed into a prolonged struggle between Monomoy Girl and Wonder Gadot, with the latter gaining the advantage approaching the final furlong.

[11] Wonder Gadot's jockey John Velasquez lodged an objection to the winner on the grounds of interference in the straight, but the result was allowed to stand.

[12] Five weeks after her win at Churchill Downs, Monomoy Girl started odds-on favorite for the Grade I Acorn Stakes over one mile at Belmont Park.

"[13] On July 22, Monomoy Girl earned her fourth Grade I win in a row in the Coaching Club American Oaks at a distance of 1+1⁄16 miles at Saratoga.

Turning for home, Geroux urged her forward and Monomoy Girl responded by opening up down the stretch for a three length win.

Monomoy Girl and Separationofpowers rated just behind the early pace set by Jump Ruler, while Midnight Bisou raced a few lengths further back in fifth.

Swinging wide around the turn, Monomoy Girl moved to the front, opening up a 1+1⁄2 length lead in midstretch despite drifting in towards the rail.

Closing steadily down the stretch, Mike Smith on Midnight Bisou first guided his filly to the rail then moved her to the outside to get around Monomoy Girl.

"[16] Monomoy Girl faced older fillies and mares for the first time in the Breeders' Cup Distaff, held on November 3 at Churchill Downs.

[17] Breaking from the outside post, Monomoy Girl stalked the early pace set by Wonder Gadot then took command rounding the turn, then held off a late run from Wow Cat to win by a length.

[19] Monomoy Girl's four-year-old season was postponed when she arrived at Churchill Downs by van from Fair Grounds with a mild case of colic at the end of March.

[20] She re-entered training in an attempt to defend her title in the Breeder's Cup Distaff, but then pulled a gluteal muscle in a workout on September 7, sidelining her again.

[21] Monomoy Girl made her comeback on May 16, 2020, at Churchill Downs as the 1:2 favorite in an allowance optional claiming race over one mile on a sloppy surface.

Her main rival was the three-year-old filly Swiss Skydiver, who had defeated Authentic in the Preakness Stakes by a short neck on October 16.

[30] Spendthrift decided to keep her in training for her six-year-old season and entered a lease-to-race agreement with MyRacehorse Stable, which in turn offered micro-shares of her to the general public.

[33] Monomoy Girl's six-year-old debut was postposed when Oaklawn Park had to cancel racing in mid-February due to severe winter weather.

[35] Monomoy Girl's next start was on April 17 in the Apple Blossom Handicap at 1+1⁄16 miles, which was billed as a match race against Swiss Skydiver.

Monomoy Girl was the odds-on favorite in the field of six, with Swiss Skydiver at 2:1 and Letruska, a multiple stakes winner who had been the 2019 Mexican champion three-year-old filly, at 7:2.

[37][36] Monomoy Girl did not bounce back from the race as quickly as expected and on May 8, it was announced that she had been sidelined due to minor muscle pains and soreness in her hamstrings.

In the statement, trainer Brad Cox said that Monomoy Girl had suffered a non-displaced sesamoid fracture during training at Churchill Downs the previous day.

[39][40] With earnings of more than $4.7 million in her career, Monomoy Girl retired as North America's fifth highest-earning female on dirt behind Midnight Bisou, Zenyatta, Beholder and Royal Delta.