As a yearling in September 2018 he was consigned to the Tattersalls Ireland sale and was bought for €26,000 by the trainer Johnny Murtagh,[2] acting on behalf of Liam Clarke's Kildare Racing Club partnership.
The early part of the flat season was disrupted by the COVID-19 pandemic and Sonnyboyliston did not return to the track until 19 June when he started second favourite for a handicap race over one mile at Gowran Park.
Ridden by the apprentice jockey Ben Coen he recorded his first success as he took the lead in the last quarter mile and won by one and a half lengths from Meaningful Star.
[8] In July he finished third in a ten furlong handicap at Navan and then carried 133 pounds to victory in a similar event at the Curragh catching the filly Thundering Nights on the line to win by a short head.
On 18 June at Limerick Racecourse Sonnyboyliston started 6/5 favourite ahead of Sunchart for the Listed Martin Molony Stakes over one and a half miles on good ground in which he was ridden by Shane Kelly.
He raced in mid-division before moving up to join the leaders in the last quarter mile, overtook Sunchart a furlong out and stayed on well to win by one and a half lengths.
[13] On 10 July Sonnyboyliston ran for the second time in England but failed to reproduce his best form as he came home sixth behind Hukum in the Group 3 John Smith's Silver Cup Stakes over one and three quarter miles at York Racecourse.
Twilight Payment reeled in the tiring front-runner in the straight but was soon challenged by Sonnyboyliston who took the lead approaching the final furlong and kept on well to win by three quarters of a length.