[2] Born in Desio, Sportiello finished his graduation with Atalanta's youth system, and subsequently moved to Seregno in a co-ownership deal.

[6] After being a regular starter during the promotion campaign to Serie B, Sportiello returned to Atalanta, being third-choice (behind Andrea Consigli and Giorgio Frezzolini).

[8] After Consigli's departure to Sassuolo, Sportiello was made starter for the 2014–15 campaign, overtaking veterans Frezzolini and new signing Vlada Avramov.

On 19 September 2023, Sportiello stepped in for an injured Mike Maignan and made his official AC Milan debut in a Champions League group stage match against Newcastle United, which concluded in a 0–0 draw.

[16] Sportiello returned on the bench by 20 January 2024, the away Serie A game against Udinese, and made six more appearances throughout the rest of the season as Mike Maignan continued to struggle with injuries in April and May of the year.