1961/1962 National Hunt season

Arkle's first racecourse appearance was on 9 December 1961 at Mullingar, where he came third of seventeen in the Lough Ennel Plate, a bumper. It was a race for amateur riders, and he was ridden by the Hon. Mark Hely-Hutchinson, son of Lord Donoughmore (one of Dreaper's major owners). On St Stephen's Day 1961, Arkle ran in another bumper, at Leopardstown. Again ridden by Hely-Hutchinson, he came fourth of ten.^[1]: 29-30

In January 1962, Arkle had his first race over jumps in the Bective Novice Hurdle over three miles at Navan. Stable jockey Pat Taaffe had elected to ride the favourite Kerforo and the ride on Arkle was taken by stable lad Liam McLoughlin. Arkle defeated 26 runners to win by 1+1⁄2 lengths, prompting his trainer to say "I think we've got something there!" In his next race, the Rathconnel Handicap Hurdle over two miles at Naas, he was ridden for the first time by Taaffe. Starting at 2/1 favourite in a field of ten, he won by four lengths.^[1]: 30-31

The season ended with two defeats in April 1962. Arkle, ridden by McLoughlin, finished unplaced in the two-mile Balbriggen Hurdle at Baldoyle, the only time in his career he would finish outside the first four. The pair then finished fourth in the New Handicap Hurdle at Fairyhouse, before Arkle went to spend the summer on his owner's farm at Bryanstown House, Maynooth, County Kildare.^[1]: 31-33

1962/1963 National Hunt season

The five-year-old Arkle started the 1962/1963 season with two wins over hurdles in Ireland, ridden on one occasion by Taaffe and on the other, when Taaffe was unable to make the weight, by his regular work rider, Paddy Woods. On 17 November 1962, Arkle ran for the first time over steeplechase fences in the Honeybourne Chase at Cheltenham. Ridden by Taaffe, who would go on to ride him in all his subsequent races over fences, Arkle started at 11/8 favourite and won by 20 lengths. It was his first race in England, and his victory earned him a mention in The Times, where he was described as a "promising type".^[1]: 35-36 As a preparation for the 1963 Cheltenham Festival, Arkle was entered in the Milltown Novice Chase at Leopardstown in February. He went off as 1/2 favourite and won by eight lengths.^[1]: 38 The race was later renamed the Arkle Novice Chase in his honour.

In March 1963, Arkle made his first appearance at the Cheltenham Festival. Having gone off as 4/9 favourite in the Broadway Novices' Chase, he put on a turn of speed from the second last, and won by twenty lengths. The season ended with two more victories in Ireland before Arkle went on his customary summer break at Bryanstown.^[1]: 38

1963/1964 National Hunt season

Arkle started the season with a win in the Donoughmore Plate, a flat race at Navan, in which he was ridden by former champion jockey T. P. Burns. There followed an easy victory in the Carey's Cottage Handicap Chase at Gowran Park, before he set off for England and a much-anticipated first meeting with the 1963 Gold Cup winner Mill House in the Hennessy Gold Cup at Newbury.^[1]: 40 In a field of ten, Mill House went off as 15/8 favourite and conceded 5 lbs to Arkle, who started at 5/2. Arkle was just a length behind the leader Mill House at the third last, when he slipped after landing and took a while to recover his momentum, finishing in third place.^[1]: 44-45

There were three victories for Arkle in Ireland, in the Christmas Handicap Chase, the Thyestes Handicap Chase and the Leopardstown Handicap Chase, before another meeting with Mill House in the 1964 Cheltenham Gold Cup. Arkle avenged his previous defeat by beating Mill House (who had won the race the previous year) by five lengths to claim his first Gold Cup at odds of 7/4 with another previous winner and top-class chaser, Pas Seul, twenty-five lengths back in third place, and Kings Nephew, the only horse to have previously beaten Mill House, last of the four runners. It was the last time he did not start as the favourite for a race. Only two other horses entered the Gold Cup that year – and that became the norm for the next two years, for there was no point in taking on Arkle at level weights – so the remaining runners were racing for place money only.^{[1]: 59-62-45}

Three weeks after his victory at Cheltenham, Arkle ran in the Irish Grand National at Fairyhouse. The racing authorities in Ireland took the unprecedented step in the Irish Grand National of devising two weight systems – one to be used when Arkle was running, and one when he was not. Arkle won the 1964 race by only one length, but he carried 30 lbs more than his rivals.^[1]: 70

1964/1965 National Hunt season

After a warm-up race in Ireland, Arkle returned to Newbury to avenge his defeat by Mill House in the previous year's Hennessy Gold Cup. In a field of nine, Arkle started at 5/4 favourite, with Mill House at 15/8. Taaffe let Arkle go to the front and jump alongside Mill House, who had nothing left as they entered the straight. Arkle strode away to win by ten lengths, with Mill House beaten into fourth place.^[1]: 76-77 Just a week later, Arkle was running in the Massey Ferguson Gold Cup at Cheltenham. He was beaten a length into third place by the grey mare Flying Wild, to whom he was conceding 32 lbs.^[1]: 80-81

Arkle returned to Ireland to win the Leopardstown Handicap Chase for a second time before retaining his crown in the 1965 Cheltenham Gold Cup, this time beating Mill House by twenty lengths. His final race of the season was the Whitbread Gold Cup at Sandown Park. In spite of giving away at least 35 lbs to his six rivals, Arkle won by five lengths and netted the largest prize of his career to date, £8,230.^[1]: 86-88

1965/1966 National Hunt season

Arkle was unbeaten in his five starts in the 1965/1966 season, winning the Gallaher Gold Cup at Sandown, the Hennessy Gold Cup for a second time, the King George VI Chase by a distance at Kempton, the Leopardstown Handicap Chase for the third time (beating Height O'Fashion by a neck conceding 42lbs in Arkle's first photo finish decider ^[2]), and the Cheltenham Gold Cup for the third time.

In the 1966 renewal, he was the shortest-priced favourite in history to win the Cheltenham Gold Cup, starting at odds of 1/10. He won the race by thirty lengths, hard held, from Dormant and just three other rivals, despite a mistake early in the race where he ploughed through the eleventh fence, while looking at his adoring fans in the stands. However, it did not stop his momentum, nor did he ever look like falling.^{[1]: 118-121} Arkle had a strange quirk in that he crossed his forelegs when jumping a fence.

1966/1967 National Hunt season

In November 1966, Arkle returned to Newbury to attempt a third consecutive victory in the Hennessy Gold Cup. Without a preparatory race, having not run for eight months and having suffered a setback in training two weeks earlier, faced five rivals all of whom carried at least 33lbs less than him. It was outsider Stalbridge Colonist, receiving 35lbs, who challenged Arkle on the run-in and won by half a length. The scale of the task Arkle faced is shown by the race-fit winner coming a close second and third in the two following Cheltenham Gold Cups, while in third place receiving 33lbs was the future 1969 Gold Cup winner, What A Myth. It was Arkle's first defeat in nearly two years; racing commentator Peter O'Sullevan described it as "a magnificent, noble effort".^{[1]: 132-136}

Arkle returned to his winning ways eighteen days later, when he landed the SGB Handicap Chase at Ascotcarrying his usual top weight of 12st 7lbs. He actually conceded 35lbs to Vultrix, himself a top-class chaser who had won the race the previous year carrying top weight of 12st 1lb, and Arkle destroyed him by 15 lengths, at which his trainer, Ryan Price, remarked that "Arkle is a living phenomenon". On 27 December 1966, Arkle ran in the King George VI Chase at Kempton Park but struck the guard rail with a hoof when jumping the open ditch, which resulted in a fractured pedal bone; despite this injury, he completed the race and was only overtaken on the run-in to finish second, a half-length behind the winner, Dormant, to whom he was still conceding 21lbs.^{[1]: 144-148} Visibly lame, Arkle was loaded into the horse-ambulance and taken back to the racecourse stables, where he remained for two months, his injured leg encased in plaster.^[1]: 151