Parts of this article (those related to BaltimoreLink Bus Route Changes) need to be updated. Please help update this article to reflect recent events or newly available information.(June 2017)
Route 21 was electrified in 1895 as a streetcar line along Preston and Caroline Streets. In 1938, the line was converted to a rubber tire bus service.[2]
In 1957, the line was extended along Carey Street to the current loop at Cumberland and Carey Streets. The line did not have any routing changes until February 8, 2009, when it was extended north to the Mondawmin Metro Subway Station along Pennsylvania Avenue.[3]