The Yankees defeat the Mariners, 6 – 1. Relief pitcher Mike Stanton makes his first major league start for New York, ending his record streak of 552 consecutive relief appearances prior to his first start. The previous record of 443 was set by Gary Lavelle of the San Francisco Giants.

On May 9, 1999 — The Yankees defeat the Mariners, 6 – 1. Relief pitcher Mike Stanton makes his first major league start for New York, ending his record streak of 552 consecutive relief appearances prior to his first start. The previous record of 443 was set by Gary Lavelle of the San Francisco Giants.

Source:
Baseball Reference May 9