Slattery was promoted to the senior squad of Kitchee ahead of the 2023–24 season following good performances in the Under-18's Youth Premier League.

[8] Slattery would also be included in the travelling squad for the 2023–24 AFC Champions League campaign, marking his first professional away outing.

[12][13] Slattery made his Hong Kong U-20 debut against Thailand in a friendly where he impressed for a good 90 minutes playing as a striker.

He was then called up for the Hong Kong U-23 where he scored a goal against Tajikistan U-23 and stole a draw from the opponents late in the game.

[14] Slattery participated in the 2023 East Asian Youth Games in Ulaanbaatar where he managed to fend off Mongolia as he scored another late goal to secure the win for Hong Kong.