1979: two-year-old season

Posse made his only appearance as a two-year-old in October 1979 when he contested the Houghton Stakes over seven furlongs at Newmarket Racecourse. Starting at odds of 14/1 he produced a strong late run but showed his inexperience ("ran very green")^[5] in the final furlong and finished second, beaten two and a half lengths by the Irish-trained Night Alert with the future Epsom Oaks winner Bireme three lengths back in third.^[6]

1980: three-year-old season

On his three-year-old debut, Posse was sent to Newbury Racecourse in England for the Greenham Stakes, an important trial race for the 2000 Guineas. In an exceptionally strong field for a Group Three event he finished strongly to take third, beaten a neck and half a length by Final Straw and 1979's leading juvenile Monteverdi.^[7] The Middle Park Stakes winner Known Fact finished fourth with the William Hill Futurity winner Hello Gorgeous in fifth. On 3 May Posse started at odds of 12/1 for the 2000 Guineas over Newmarket's Rowley Mile with the undefeated French colt Nureyev being made the 13/8 favourite. Eddery tracked the leaders on Posse and appeared to have a good chance three furlongs from the finish when he was badly hampered and almost fell when Philippe Paquet on Nureyev barged his was through the centre of the field to make his challenge.^[8] Posse recovered well and made up a great deal of ground in the final furlong to finish third, beaten a neck and three-quarters of a length by Nureyev and Known Fact. An inquiry by the racecourse stewards ruled that Paquet had been guilty of "reckless" riding and, under the rules of racing at the time, had no option but to disqualify the favourite and relegate him to last place. The race was awarded to Known Fact, with Posse being promoted to second.^[9] Two weeks later, Posse started 7/2 second favourite behind Monteverdi for the Irish 2,000 Guineas at the Curragh, despite having performed poorly in a training gallop. He appeared the likely winner two furlongs from the finish but eventually finished fourth behind Nikoli, Last Fandango and Final Straw, beaten less than a length. While some observers felt that Posse had failed through lack of courage, Timeform suggested that the colt had not recovered sufficiently from his exertions at Newmarket.^[9]

Posse reappeared in the St James's Palace Stakes at Royal Ascot in June. Shortly before the race a half-share in the colt was sold for $1 million to the Derisley Wood Stud, despite the fact that he was still a maiden after four races. At Ascot he was again opposed by Last Fandango and Final Straw, but the lightly-raced Dalsaan was made the odds-on favourite, with Posse starting the 11/2 third choice in the betting. Eddery rode the colt with great confidence, restraining him towards the back of the field and he was still only in fifth place with a furlong and a half to run. Posse then moved up on the outside, and produced what Timeform described as a "dazzling turn of speed"^[9] to overtake Final Straw well inside the final furlong and win very easily by one and a half lengths. In the Sussex Stakes at Goodwood Racecourse in July, Posse was matched against older horses for the first time, but Final Straw appeared to be his most serious opponent and he was made the 8/13 favourite. Eddery again held the colt up for a late challenge but in the straight he appeared unlikely to obtain a clear run as he was trapped on the inside. In the closing stages Posse was switched to the outside and again produced a striking burst of acceleration to catch Final Straw in the last stride and win by a head.^[9] There was considerable anticipation of a race between Posse and either Known Fact or Kris the leading miler of 1979, both of whom had missed the summer with training problems. Shortly after his win at Goodwood however, Posse contracted a respiratory infection^[10] from which he failed to recover sufficiently for him to take part in the major autumn races. He did not run again and was retired to the Derisley Wood Stud.^[11]