1998: two-year-old season

Mus-If made his racecourse debut in a maiden race over six furlongs at Leopardstown Racecourse on 18 June in which he started at odds of 7/2 and finished second by a neck to Fable after leading for most of the way. Eighteen days late he recorded his first success in a maiden over seven furlongs at the Curragh. Ridden by Mick Kinane and starting the 6/4 favourite, he took the lead three furlongs out and kept on well in the closing stages to win by a head from Globe Theatre.^[6] The colt was moved up in class for the Listed Tyros Stakes over the same course and distance on 15 August for which he started favourite but was beaten into second place by the Jim Bolger-trained filly Arctic Blue. He found another Bolger filly, St Clair Ridge, too good for him in the Group 3 Futurity Stakes two weeks later and was beaten a length and a half into second place.

In the Group 1 National Stakes over seven furlongs at the Curragh on 20 September Mus-If, with Kinane in the saddle started the 8/1 third choice in the betting behind the Acomb Stakes runner-up Coliseum and Globe Theatre. None of the other six runners had contested a Group race, making the field appear to be a poor one by Group 1 standards.^[7] The best horse in the race was probably the British-trained Royal Rebel, who later won two editions of the Ascot Gold Cup. Wearing blinkers, Mus-If went to the front from the start, opened up a clear advantage approaching the last quarter mile and held off the late challenge of Coliseum to win by a neck.^[8]

1999: three-year-old season

Mus-if began his second season in the Listed 2000 Guineas Trial Stakes over one mile at Leopardstown on 18 April. He took the lead in the straight but was caught in the final strides and was beaten a head by the Aidan O'Brien-trained favourite Saffron Walden.^[9] In the Irish 2000 Guineas five week later at the Curragh he went to the front two furlongs out but was overtaken entering the final furlong and came home fourth behind Saffron Walden, Enrique and Orpen with the 2000 Guineas winner Island Sands in fifth. The colt was the dropped in class for the International Stakes over nine furlongs at the Curragh in June but finished last of the five runners behind the Henry Cecil-tranied favourite Great Dane. On his final European start, Mus-If was sent to Germany to contest the Group 2 Grosse Europcar-Meile at Cologne-Weidenpesch Racecourse on 25 September. He was made the 3/5 favourite but came home fourth behind the four-year-old El Divino.

2001: five-year-old season

At the end of his European racing career Mus-If was sent to the United States where he was "mistakenly" gelded.^[10] He re-entered training in 2001 and joined the stable of Neil Drysdale in California. He raced twice at Santa Anita Park, finishing second in a claiming race in February and fourth in a similar event in March.^[11]

2002: six-year-old season

In 2002 Mus-If moved to the training stable of Michael Pino and began his campaign in claiming races at Fair Grounds Race Course. After finishing fourth and third on his first two starts he recorded his first success for three and a half years when taking a claimer on 11 March. After winning a similar event at Keeneland Racecourse in April he was stepped up in class to contest the Henry S. Clark Stakes over one mile at Pimlico Race Course on 4 May. Starting favourite against ten opponents he took the lead in the stretch and won by five lengths under a "hand ride" from his trainer's brother Mario Pino.^[12] Three weeks later at the same track, Mus-If extended his winning sequence to four in the C. Edmund O'Brien Stakes. With Mario Pino again in the saddle he started at long odds-on and came home two lengths clear of Cynics Beware after taking the lead on the home turn.^[13] His victory was only confirmed after a lengthy inquiry by the racecourse stewards into possible interference caused by Mus-If to one of his rivals on the far turn.^[14] The gelding's winning streak came to an end in June when he was beaten into second place by Del Mar Show in the Grade III New Hampshire Sweepstakes Handicap at Rockingham Park.

In the late summer and autumn of 2002 Mus-If was campaigned at Delaware Park Racetrack where he finished sixth in the Sussex Handicap and third in the Leader of the Band Stakes before winning a claiming race on 6 October. On his last two starts he finished second in a claiming race at Churchill Downs in November and seventh in a similar event at Fair Grounds on 8 December.^[11]