St. Ivany played youth hockey for the Los Angeles Jr. Kings, and then played junior hockey for the
Sioux Falls Stampede of the
United States Hockey League (USHL). St. Ivany was selected by the
Philadelphia Flyers in the fourth round, 112th overall, of the
2018 NHL entry draft. In the
2021–22 season, St. Ivany was named a
Hockey East Third-Team All-Star. At the conclusion of his four-year collegiate career, St. Ivany became a free agent on August 15, 2022. On August 20, 2022, St. Ivany signed a two-year, entry-level contract with the
Pittsburgh Penguins. St. Ivany made his NHL debut on March 22, 2024, in Pittsburgh's game against the
Dallas Stars. On April 2, 2024, St. Ivany recorded his first NHL point, assisting on an
Evgeni Malkin goal against the
New Jersey Devils. On May 21, 2024, St. Ivany re-signed with Pittsburgh on a three-year contract. On October 2, 2025, the Penguins announced St. Ivany had suffered a lower-body injury, and would miss a minimum of six weeks. ==International play==