[1] Mackin joined the band in 1997, from high school friends Ben Harper and Longineu W. Parsons III.

As Ryan's friend Dan McLintock from Jacksonville pop punk band Inspection 12 was still with Craig's Brother at the time and they were currently in the studio recording their second album Lost at Sea, Sean was drafted to record violin for their song "Back and Forth".

Shortly after the release of their EP Todd Clary left the band, Key then filled in as the rhythm guitarist.

Yellowcard then began the recording of their major-label debut album Ocean Avenue in late 2002, gaining the band major success.

Billboard 200, the album was a commercial success in the United States with the hit singles Way Away, Ocean Avenue, and Only One.

Lead guitarist Ben Harper left the band in April after finishing guitar tracks for the album.

The band spent the rest of the year promoting their new album nonstop since its release while touring with Linkin Park and Blue October.

The band needed to reschedule several dates during their European leg of their tour in January while Parsons went home to spend time with family.

In April 2008 Yellowcard officially announced an "indefinite hiatus" canceling their European and acoustic tours.

[4] In response to Mackin's announcement regarding his diagnosis, a group of fans started the Strong for Sean project.