First opening for revenue service in 2001, the St. Clair extension in St. Clair County, Illinois, was the first MetroLink extension beyond the original line.  The 2001 extension (Phase 1) to Southwestern Illinois College added 19 miles and 8 stations to the MetroLink system.  A further 2003 extension (Phase 2) to Shiloh-Scott Airforce Base added another 5 miles for the new terminal station.The 1991 Systems Analysis prioritized the St. Clair corridor as a Tier I priority. In 1995, an alternatives analysis and draft environmental impact statement was completed and the light rail project advanced into preliminary engineering.  In 1996, the full funding grant agreement was signed for the St. Clair extension (Phase 1) to College in Belleville, Illinois, utilizing $244 million in federal New Starts funding or 72 percent of the construction cost. The total construction cost to the College was $339 million, with the local match of $95 million funded by the St. Clair County Transit District.  The County's share came from a ½ cent sales tax, which the voters had approved in 1994 for MetroLink expansion. In 1998, engineering and design were complete on the MetroLink system's first extension, and the St. Clair line opened for revenue service in 2001.

The remainder of the St. Clair corridor (Phases 2 and 3) to Mid-America Airport in Mascoutah, Illinois, as described in the AA/DEIS evaluated in 1995, has only been partially built. A five-mile, one-station extension from College to Shiloh-Scott opened in 2003. This $75 million extension was actually the first in the St. Louis region not to use federal funds, with a $60 million grant from Illinois' then FIRST program (Fund for Infrastructure, Roads, Schools and Transportation), and $15 million from the St. Clair County Transit District.

The final piece of the St. Clair corridor (Phases 2 and 3) beyond Shiloh-Scott to Mid-America Airport has completed the planning stage but has yet to enter into design due to lack of funding for implementation.