1977: two-year-old season

More So made her racecourse debut in a seven furlong maiden race at the Curragh in September 1977. Starting at odds of 12/1 in a thirteen-runner field she won by three quarters of a length from the Vincent O'Brien-trained colt Encyclopedia.^[3]

1978: three-year-old season

On her first appearance as a three-year-old More So was ridden by Christy Roche when she was one of 17 fillies to contest the Irish 1000 Guinea on good to firm ground at the Curragh on 12 May. Despite her lack of experience she was made the 2/1 joint favourite alongside Cistus, the only British-trained challenger who had won the Waterford Candelabra Stakes and finished second in the Critérium des Pouliches in the previous year. The best fancied of the other runners were Enid Calling (winner of the Tyros Stakes and Mulcahy Stakes), Quatemala (second in the Athasi Stakes) and Ridaness (Moyglare Stud Stakes). The race was run only a week after the English 1000 Guineas and Enid Calling (seventh to Enstone Spark at Newmarket) was the only filly to contest both races. After tracking the leaders, More So overtook Ridaness to gain the lead inside the final furlong and kept on well to win by one and a half lengths. Sorbus (later to be disquallifed after "winning" the Irish Oaks) finished strongly to take second by a short head from Ridaness with Sutton Place in fourth.^[2]

Shortly after her win at the Curragh the filly contracted a viral infection and missed an intended run in The Oaks reappearing in the Coronation Stakes at Royal Ascot later in June. She started favourite but looked outpaced and finished fifth, six lengths behind Sutton Place, who won by a short head from Ridaness. In late July she returned to Ireland and matched against male opposition in the Desmond Stakes over one mile at the Curragh. She won by two lengths from Ridaness with Strong Gale (runner-up to Jaazeiro in the Irish 2000 Guineas) four lengths back in third. At York Racecourse in August she was moved up in distance to contest the seventh running of the Benson and Hedges Gold Cup over ten and a half furlongs and finished fifth behind Hawaiian Sound. On her final start of the year she was made 5/2 third favourite for the Joe McGrath Memorial Stakes at Leopardstown Racecourse on 23 September but ran poorly and finished sixth of the seven runners behind Inkerman.^[2]

1979 & 1980: North American racing career

Before the start of the 1979 season More So was bought privately by Nelson Bunker Hunt and was exported to the United States where she was trained by Charlie Whittingham. More So was unplaced in her first three American starts before finishing second in an allowance race at Hollywood Park Racetrack and then running third to Giggling Girl in the Beverly Hills Handicap at the same course on 1 July. She recorded her first win in the United States in an allowance at Hollywood Park two weeks later and followed up with a victory in the Palomar Handicap at Del Mar Racetrack on 11 August, beating Giggling Girl and Wishing Well. She finished second to Country Queen in the Ramona Handicap at Del Mar in September and was then stepped up to Grade I level and finished fifth in the Yellow Ribbon Stakes at Santa Anita Park on 3 November. She recorded her third win of the year when she took the Children's Hospital Handicap at Bay Meadows two week later and ended her season by finishing fourth in the California Jockey Club Handicap at the same track in December.^[5]

More So remained in training as a five-year-old in 1980 but did not recapture her best form and failed to win in twelve starts. Her only placed effort came on 31 July when she finished third in an allowance race at Del Mar.^[5]