2016: three-year-old season

Harbour Law made his racecourse debut in a maiden race over one and half miles on the synthetic Polytrack surface at Lingfield Park Racecourse on 18 March. Ridden by Sean Levey he started a 20/1 outsider and finished second, one and a half lengths behind Shoofly and twenty lengths clear of the other three finishers. Harbour Law was subsequently moved to the stable of Laura Mongan at Epsom. On 1 May the colt made his first appearance on turf when he started at odds of 4/1 in a maiden over the same distance at Salisbury Racecourse. Ridden by George Baker who became his regular jockey, he took the lead two furlongs out and won by two and three quarter lengths from Forth Bridge.^[6] Eighteen days after his win at Salisbury Harbour Law was moved up in distance for a handicap over fourteen furlongs at Sandown Park Racecourse in which he was allotted top weight of 133 pounds. He disputed the lead from the start and drew ahead of his opponents in the closing stages to win by one and a half lengths.^[7]

On 17 June, the colt was moved up in class and distance for the Listed Queen's Vase over two miles at Royal Ascot. Starting at odds of 8/1 in an eighteen-runner field he was amongst the leaders from the start and stayed on well in the straight to finish second, three quarters of a length behind the 33/1 outsider Sword Fighter. On 7 July Harbour Law contested the Group 3 Bahrain Trophy over thirteen furlongs at Newmarket Racecourse. After taking the lead approaching the last quarter mile he was overtaken in the closing stages and finished fourth of the nine runners behind the Aidan O'Brien-trained Housesofparliament.^[8]

On 10 September Harbour Law, partnered by Baker started a 22/1 outsider in a nine-runner field for the 240th running of the St Leger Stakes over fourteen and a half furlongs at Doncaster Racecourse. Idaho started the odds-on favourite whilst the other runners included Sword Fighter, Housesofparliament, Ventura Storm (Prix de Reux) and the John Gosden-trained Muntahaa. After racing towards the rear of the field Harbour Law began to make steady progress in the straight, by which point Idaho had unseated his jockey. Housesofparliament and Ventura Storm appeared to have the race between them but Harbour Law produced a strong late run to gain the advantage in the final strides and won by three quarters of a length. Ventura Storm took second by a short head from Housesofparliament with a gap of ten lengths back to Muntahaa in fourth. Laura Monagan, who became the first woman to train a St Leger winner said "It was brilliant. I think I screamed a lot. I'm in shock. I'm so glad we came here and he's proved that he can do it. We didn't want to ride him like that, but George knows what he's doing and all credit to him. We knew we could leave it to him. It was a really professional performance by the horse, we knew he'd go somewhere in life. It proves we can do it with the ammunition. It's great to have a horse like this and bring him here for a race like this".^[9]

2017: four-year-old season

Harbour Law began his second campaign in the Group 3 Sagaro Stakes over two miles at Ascot on 3 May. Ridden by Tom Queally he never looked likely to challenge and came home last of the seven runners behind the five-year-old mare Sweet Sensation. On 22 June 2017 at the same track Harbour Law was partnered by Jim Crowley when he went off a 33/1 outsider for the Group 1 Ascot Gold Cup over two and half miles ran. He was always among the leaders, moved into second place three furlongs from the finish and despite being unable to make further progress he stayed on well to finish third behind Big Orange and Order of St George.^[10]

On the day after the Gold Cup Harbour Law was found to have sustained a tendon injury which kept him off the course for the rest of the year.^[11] He remained in training the following year but never raced again.